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Car Forum / Toyota / Toyota Cars / November 2009wheel circumference
Is there any difference in speedometer/odometer performance between wheels that have the same overall circumference but one has thin sidewalls and the other has wide sidewalls? I know there is a handling performance and comfort difference. I've always preferred the looks of the extra wide sidewalls with small wheel hubs to the more popular huge wheels with thin tires. > Is there any difference in speedometer/odometer performance between > wheels that have the same overall circumference but one has thin > sidewalls and the other has wide sidewalls? I know there is a handling > performance and comfort difference. I've always preferred the looks of > the extra wide sidewalls with small wheel hubs to the more popular huge > wheels with thin tires. Can you make a guess as to who buys these wide tires with the fancy spoke-less rims? Youth, that's who. The same youth that spends lots of money on their cell phones, ring tones, text messaging etc, but refuses to buy healthcare and demands government buy it for them or their parents coverage extended to cover them till age 30??????? Nancy pea feels sorry for these same youth that can't handle money yet. I have little mercy on the wild spenders. Having said that, there are responsible youth out there too. -- > Is there any difference in speedometer/odometer performance between > wheels that have the same overall circumference but one has thin > sidewalls and the other has wide sidewalls? I know there is a handling > performance and comfort difference. I've always preferred the looks of > the extra wide sidewalls with small wheel hubs to the more popular huge > wheels with thin tires. Yes, there would be a difference in speedometer and odometer calibration. The operative wheel/tire radius is not the nominal overall radius as measured from hub center to tire tread, but a smaller "working" radius line that is traced from the hub center to the /road/. Notice how the tire bulges at the bottom? That's where the working radius line ends. This working line is much shorter than the nominal radius line, resulting in a much smaller circumference than nominal. That's how ABS- based low-tire-pressure systems know which tire has low pressure. The shorter the tire sidewall (as for those larger wheels), the stiffer the sidewall, the less the bulge at the bottom of the tire and thus the less the reduction between nominal radius and the working radius.  SignatureTegger ----- Original Message ----- From: "Tegger" <invalid@invalid.inv> Newsgroups: alt.autos.toyota Sent: Wednesday, November 04, 2009 7:26 AM Subject: Re: wheel circumference >> Is there any difference in speedometer/odometer performance between >> wheels that have the same overall circumference but one has thin [quoted text clipped - 19 lines] > sidewall, the less the bulge at the bottom of the tire and thus the less > the reduction between nominal radius and the working radius. While agree that there may be a small difference (a very small difference) in rolling diameter for tires of the same overall diameter but with different inside (wheel) diameters, I don't entirely buy you explaination of why. For sure you are right about how the ABS based low tire pressure sensors work, but they take miles of driving to detect a very significant difference in tire pressure. BUT..... Modern radial tires are not like hard wheels, they are like tank treads. The rolling diameter is mostly based on the diamter of the steel belt in the tire as long as the tires are properly inflated. This belt has to make one revolution per revoltion of the tire. I know this is obvious, but think about the implication. There is very little stretch in the steel belt. Now think about how this relates to the tank tread analogy....Even though the tire flattens out at the road surface, every portion of the belt still passes over the road in a almost exactly a 1 to 1 ratio. This would only be false if the belts stretched a lot - which they don't or if they took a really strange wavy or concave shape at the road interface, which also doesn't happen - at least if the tire is anywhere close to properly inflated. Any how back to the original question: Here is a chart for Michelin Pilot Sport Tires that relates tire size, overall diameter, and revolutions per mile: Revolutions Overall Per Tire Size Diameter Mile 245/40ZR17 24.7" 841 225/45ZR17 24.8" 837 255/35ZR18 25.2" 824 225/40ZR18 25.3" 821 265/35ZR18 25.3" 819 255/40ZR18 25.9" 770 285/35ZR18 25.9" 804 255/45ZR17 26" 798 255/45ZR17 26" 798 255/40ZR18 26" 800 285/35ZR18 26" 800 225/40ZR19 26.1" 797 265/40ZR18 26.6" 782 245/45ZR18 26.8" 780 245/45ZR18 26.8" 780 245/45ZR18 26.8" 775 255/45ZR18 26.9" 771 285/40ZR18 26.9" 771 235/50ZR18 27.2" 762 In general I don't think you can decide that tires with the same overall diamter but different wheel sizes will have greater differences in rolling diameters than tires of the same size but different brands or models (construction techniques). In any case the difference is very small (1% or less). Ed > While agree that there may be a small difference (a very small > difference) in rolling diameter for tires of the same overall diameter [quoted text clipped - 8 lines] > treads. The rolling diameter is mostly based on the diamter of the > steel belt in the tire as long as the tires are properly inflated. I think you need to go do some actual observation and measurement. Go outside and measure those distances on your own tires. I'll use my own car's front tires as an example for illustration: My tire size is 195/60-14. That gives me a nominal diameter of 23.21". An actual (as best as I can eyeball) diametrical measurement reveals 23", when measured across the unloaded portion of the tire from front-to-back. That means the unloaded radius is 11.5". If I measure from dead-center of the hub to the road (the loaded, or "working" radius) however, I get 10.625". That's 7/8" difference, or about 8% less than the unloaded radius. Now... Consider my REAR tires. The car has 61-39 front/rear weight distribution. Same tire, same pressure, much lighter loading. The working radius here I measure at 11.0625". The rear tires thus have a 4% larger working radius than the fronts. It is impossible to have a contact patch on the road unless the tire develops a "flat spot" where the tire contacts the road. This has nothing to do with the steel belt or anything else. Even a hard-rubber towmotor tire has a flat-spot and a contact patch. The center of the contact patch is the end point for that "working" radius line. If you have bigger wheels and shorter, stiffer tire sidewalls within the same unloaded diameter, the loaded distortion will be less, which means the wheel/tire assembly will have a larger working circumference. How much more? Possibly up to four or five percent, possibly as low as one or two percent. Depends. But there WILL be a difference. SignatureTegger >> While agree that there may be a small difference (a very small >> difference) in rolling diameter for tires of the same overall [quoted text clipped - 59 lines] > two > percent. Depends. But there WILL be a difference. I tried really hard to get you to see this differently. Go back and rear my prior post. Tires are not hard wheels. The revolutions per mile is not directly related to the loaded radius of the tire in the manner you are suggesting. It is likely a factor, but a very minor one.... Think! Car tires are not hard wheels like forklift tires. How can what you are saying be true? Look at it a different way...every time the tire goes through a revolution, there is apporximately a one to one relationship between the tire and the road surface (every part of the tire contact the road). One revolution of the tire will move the car forward by the circumfrence of the tire. If it doesn't you have to have slip (which will be true if you spin the tires, but is a negligible factor if you are cruising straight ahead on a level road at a moderate sped). The fact that the tire flattens out at the road surface doesn't change this. Of course rubber is flexible and can stretch/shrink, which is why the really important circumfrence is the circumfrence of the steel belts inside the tire. The treads area can stretch and flex (think about how tank treads move) but the steel belt stretchs only by tiny amounts. Unless the tire is so underinflated that the tire assumes a concave shape at the road surface, the effective rolling radius is not significantly dependednt on the distance from the road to the wheels center axis. Like most real world things, there are other factors that come into play, so I don't doubt that there is some effect on revolutions per mile related to tires of the same outside diameter with different inner diameters (i.e., wheel size), but the I don't think the loaded radius is the prime reason. One more thing to think about. If you are right, where does the extra tire go? You are suggesting that one revolution of the tire will result inthe car moving forward by a distance equal to the loaded radius of the tire times 2 times pi. But the actual circumfrence of the tire is the unloaded diameter of the tire times pi. Since 2 times the loaded radius of the tire is less than the unloded diameter of the tire times pi, this implies something is happening witht he "extra" circumfrecne in your senario. Where is it going? You might convince me that the rubber portion is stretching and shrinking to accomodate your theory, but what is happenign with the steel belt in the tire? One last analogy - think conveyor belt..... Did you look at the chart I included with the prior note? Ed >>> While agree that there may be a small difference (a very small >>> difference) in rolling diameter for tires of the same overall diameter [quoted text clipped - 88 lines] > > Ed When ONE tire is different than the others, it has to go a different speed to travel the same distance. It has to. The differential is able to handle the difference, so the car does not pull itself apart -- gross exageration, I know. But the speed sensors on each wheel will see the different speed of the different tire -- it's different because it's a different size or because the air pressure is lower, or higher I suppose. In any case, the speed sensors see the different size and treat it as a low pressure warning, reason to activate the ABS system, or the Traction Control, depending on other factors that the computer monitors. You are right, though. You posted a chart that shows a variety of tire sizes and lists equivelents that result by changing the various specs of the tires. If all tires are the same size (staggered tires -- different front and rear present complications) and the proper equivelent for the factory size, then the pressure monitor that is speed-based, the ABS and the Traction Control would all work properly. Staggered tires present complications because they are already slightly different sizes from front to rear, and altering the balance might be problematic. But if the tires remained equivelent from left to right, then the variations from front ot rear should remain a constant, and the systems should still work properly. I'm not saying there will be a problem for staggered tires setups, I'm only saying there is an added consideration -- which I've never dealt with before. "C. E. White" <cewhite3@mindspring.com> wrote in news:hcur0a$o5e$1 @news.eternal-september.org: > How can what you are saying be true? Look at it a different > way...every time the tire goes through a revolution, there is > apporximately a one to one relationship between the tire and the road > surface (every part of the tire contact the road). One revolution of > the tire will move the car forward by the circumfrence of the tire. The /working/ circumference, which changes with all sorts of factors, pressure, weight, sidewall height. As I said before, ABS-based low-pressure warning systems _could not work_ unless this was true. > If > it doesn't you have to have slip (which will be true if you spin the > tires, but is a negligible factor if you are cruising straight ahead > on a level road at a moderate sped). Not so. It's called "scrub" and is the reason tires wear even in a straight-ahead, steady-state position. > The fact that the tire flattens > out at the road surface doesn't change this. But that makes /all/ the difference. > Of course rubber is > flexible and can stretch/shrink, which is why the really important [quoted text clipped - 4 lines] > road surface, the effective rolling radius is not significantly > dependednt on the distance from the road to the wheels center axis. Then how can an ABS-based warning system work if it cannot depend on changes in working radius? And monitoring changes in rolling circumference due to changed working radius _IS_ how such systems operate. > Like most real world things, there are other factors that come into > play, so I don't doubt that there is some effect on revolutions per [quoted text clipped - 4 lines] > One more thing to think about. If you are right, where does the extra > tire go? Where did that 1/4" go? I don't know. I suspect there's a minuscule stretch or displacement around the unloaded portion of the tire. But in order to confirm that, I'd need to jack the wheel off the ground and take some really careful measurements both loaded and unloaded. My contact patch is about 4.25" front-to-back (about 45deg of rotation). On my tire, a line traced from front-to-back on that flat spot is about a quarter-inch shorter than the curve would be if the tire was not flattened by the load. If that 1/4" were distributed evenly around the unloaded portion of my tire, each of the 315 degrees of unloaded tire would have to displace or stretch about .0008", which is plausible. Plus some of the distortion would be compressed into the flat spot, so the .0008 might be an overestimate. > result inthe car moving forward by a distance equal to the loaded > radius of the tire times 2 times pi. But the actual circumfrence of [quoted text clipped - 4 lines] > that the rubber portion is stretching and shrinking to accomodate your > theory, but what is happenign with the steel belt in the tire? Well, I have one guess: The steel belt is a woven assembly with the wires on a bias. It's possible the weave distorts under compression and extension, much like rope, cloth or window screen can. > One last analogy - think conveyor belt..... > > Did you look at the chart I included with the prior note? I did, but the chart misses the point. My trump card is ABS-based low-pressure warning systems. I know for a fact that they work by sensing a low-pressure tire spinning faster than one with higher pressure, which can ONLY happen if the working radius (and circumference) can shrink and grow on an otherwise inextensible tire. SignatureTegger > "C. E. White" <cewhite3@mindspring.com> wrote in news:hcur0a$o5e$1 > @news.eternal-september.org: [quoted text clipped - 13 lines] > As I said before, ABS-based low-pressure warning systems > _could not work_ unless this was true. Very ture, but they work by comparing the response of similar tires and take a relatively long time to figure out that one is runnnig at a much lower pressure than the others (miles of driving). Lots of thing affect the number of revolutions per mile for a tire, but most are minor. Even the speed of the car has an effect. My only point is, you can't treat a tire like a hard rubber wheel. The loaded radius (distance from the axle centerline to the ground) is not the most significant factor in determining the rolling diameter of the tire. >> If >> it doesn't you have to have slip (which will be true if you spin [quoted text clipped - 26 lines] > circumference due to changed working radius _IS_ how such systems > operate. I did not say that there was no effect, just that it is a major effect. The ABS tire monitoring systems compare the response of the tires. A tire with a much lower pressure than the others will on average have a different rolling radius, but it is a very small difference. Thise systems takes miles to figure out a tire is underinflated. >> Like most real world things, there are other factors that come into >> play, so I don't doubt that there is some effect on revolutions per [quoted text clipped - 52 lines] > > I did, but the chart misses the point. The point was that the rolling circuimference of tires with similar outside diamters but different rim diamters are usually very close.. > My trump card is ABS-based low-pressure warning systems. I know for > a [quoted text clipped - 4 lines] > (and circumference) can shrink and grow on an otherwise inextensible > tire. Again, I agree that these ABS systems work by detecting changes in the rolling radius of the tire, but the effects are very small and the system take miles to figure out that the tires are different. The rolling radius is affected by many things, temeprature, speed, inflation pressure, even the particular road surface, but the ABS based systems work by comparing tires to other tires on the car. They can detect samll changes that affect one tire comapred to the others. Therefore the small effect on the rolling radius due to a large change in air pressure (>20% decrease) is detectable by these sysems I am sure you are greatly over emphasizing the contribution of the loaded radius on the rolling diamter of tires, but don't know of any other effective arguements. Maybe an experiment would convince you. If you have the time, measure the loaded radius, mark the tire, move the car for 100 revolutions of the tire, and then measure the distance moved...You will find that it moved a significantly greater distance than 2 x pi x loaded radius x 100. Ed "C. E. White" <cewhite3@mindspring.com> wrote in news:hd18tv$dr6$1 @news.eternal-september.org: > I am sure you are greatly over emphasizing the contribution of the > loaded radius on the rolling diamter of tires, but don't know of any [quoted text clipped - 3 lines] > moved...You will find that it moved a significantly greater distance > than 2 x pi x loaded radius x 100. I must be nuts, because I actually went out and tested your theory (and mine). I hope you do me the grace of actually reading this, because I did perform the test instead of being Usenet-snarky and telling you to go and do the test yourself. After making a gauge with corrugated cardboard, I discovered that the unloaded diameter of both front and rear tires is 23.25" dead-on. 23.25" x 3.14159 = 73.04" unloaded circumference. Tire pressures at time of test were all the same, 31 lbs (checked hot). On a flat, newly paved industrial parking lot, I marked the tires (and the lot) with chalk. Leaning out the window, I then slowly rolled the car so that the mark on the left tire described ten revolutions, coming down to the very bottom again. That covered almost 60 feet (100 revolutions was not practical for me.) I did this four times each, for the front and then for the rear tires (both sides). The results were very consistent. The results? Actual distance covered for the fronts: 704.5" Actual distance covered for the rears: 708.5" Now, how about the "loaded" radius? For the front left, it's 11.75", which gives a circumference of 67.54" However, that tire actually covered 70.45" per rev, not 67.54". But at the same time it was not covering 73.04" either. For the rear left, the loaded radius is 11.0625". This gives a circumference of 69.51". That tire actually rolled 70.85" in the test. Let's summarize, as percent reduction from unloaded to loaded: Front hypothetical rolling circum based on loaded radius: 7.5% less Front actual rolling circum based on test result: 3.55% less Rear hypothetical rolling circum based on loaded radius: 4.8% less Rear actual rolling circum based on test result: 3% less Looks like the fronts split the difference between unloaded and loaded, and the rears were affected pretty close to what I theorized. SignatureTegger >> While agree that there may be a small difference (a very small >> difference) in rolling diameter for tires of the same overall diameter [quoted text clipped - 44 lines] > more? Possibly up to four or five percent, possibly as low as one or two > percent. Depends. But there WILL be a difference. None of that matters to the ABS if all tires on the ground are the same size, even if the tires are not the size that is specified for the car. The ABS/Low Tire Pressure systems look at differences in the rotational speed of one tire vs. the others. If all four tires were the same, then the speed of the tires would also be the same and the ABS would function properly. The tire size _can_ influence the speed display on the speedometer, but as has been shown in several charts, if the stock size is changed for an aftermarket size that is properly equivelent, the influence over the speedometer is insiginficant, and if the new tire is a few percent larger, the actual effect on the speedometer is desirable -- it makes the speedo display the proper speed instead of display a lower speed than one is actually travelling. My car came with a 205/50x16, the car before it came with a 225/55x15, I put a set of 225/45x17s on both of these cars, and all three tires are functional equivelents for one another. (I had a car that had the 15s on it, I put on the 17s, and a lady in a double-know pants suit and blue hair slammed into me. I bought the car with the 16 on it, and move the 17s to it). The affect on the speedo was that the 17's changed the error at 80mph from almost 5mph to just under 2mph. Foremrly, the speedo would read 80 when doing 75-ish, now it reads 80 when doing 78-ish. But since all four tires are the same size, and functional equivelents, the ABS system still works right. The OP wants a larger sidewall for some reason, not a smaller one. (I don't get the logic he's using, but it's not my decision.) If he wants to go down an inch on the rims he's using, then he would go up 5% on the aspect ratio (the center figure in 190 / 55 x 15) to keep the same tread width. If he wanted to keep the same rims but get a larger sidewall, then he's have to get a thinner tire. On second thought, I'd have to run the numbers, I'm not sure he can get more sidewall on the same rim because if the width got less but the aspect ratio went up, the result would be the same sidewall -- 190 x .55 and 180 x .6 are different by 3.5mm, hardly a useful difference for any practical reason. He would get 10mm less rubber on the ground and virtually no change in the sidewall. >The OP wants a larger sidewall for some reason, not a smaller one. (I >don't get the logic he's using, but it's not my decision.) I don't want anything changed on _my_ car. I was just wondering if there is a difference between tires with thin sidewalls and those with very wide sidewalls when it comes to odometer/speedometer readings. And I'm not talking about one inch differences, more like 45s vs. 85s. After you guys work out all this technical minutia let me know what the consensus is. >>The OP wants a larger sidewall for some reason, not a smaller one. (I >>don't get the logic he's using, but it's not my decision.) [quoted text clipped - 6 lines] > After you guys work out all this technical minutia let me know what the > consensus is. You'll never get "consensus". The fact is: Given a specific sidewall height and overall tire diameter, if you change ONLY the sidewall height (bigger wheel), you WILL change the rolling circumference. If you go from a 60 to a 50, you probably won't notice the difference. If you go from an 80 to a 45, there would be a big difference. SignatureTegger > > > The OP wants a larger sidewall for some reason, not a smaller > > > one. (I don't get the logic he's using, but it's not my decision.) [quoted text clipped - 18 lines] > difference. If you go from an 80 to a 45, there would be a big > difference. So will the wheel with the 80 tire rotate more or less than the wheel with the 45? Which one will show more than the correct speed and which one will show less? Signature"If you don't read the newspapers you are uninformed; if you do read the newspapers you are misinformed." ~ Mark Twain >> > > The OP wants a larger sidewall for some reason, not a smaller >> > > one. (I don't get the logic he's using, but it's not my [quoted text clipped - 23 lines] > with the 45? Which one will show more than the correct speed and > which one will show less? The OEM wheel, tire size, and speed rating will be closest to the manufacturer's intended speedometer calibration. When it comes to aftermarket, I wish I could pin that question down for you, but I can't. The answer depends on the actual rolling circumference that each tire describes when loaded the way your car will load it. One maker's 225/45-16 won't necessarily measure the same actual unloaded diameter as another maker's 225/45-16. And those tire-shop tire-size equivalency charts are fraught with uncertainty on account of that. But: Given two differently-seriesed tires with identical unloaded circumferences, the one with the shorter sidewall will have the larger rolling circumference and the lower speedometer reading. The upshot is that the difference will likely not be enough for anybody to notice unless he's very specifically looking for that difference. Even the cops recognize that, which is partly why they give you 5mph grace (or more depending on the jurisdiction) before whacking you. SignatureTegger >>> > > The OP wants a larger sidewall for some reason, not a smaller >>> > > one. (I don't get the logic he's using, but it's not my [quoted text clipped - 26 lines] > The OEM wheel, tire size, and speed rating will be closest to the > manufacturer's intended speedometer calibration. That is not entirely true. For example, the Camry has several different tire sizes that can be fitted by virtue of the different trim levels and various option packages. The speedometer is not calibrated for each different size of tire package, it is calibrated for the largest tire package. Smaller tires will have greater speedometer error than the larger tires. If the base model Camry gets a 195 / 60 x 15, the fully loaded can have a 215 / 45 x 17, and the diameter of the tire is different by less than one-half inch. The speedo calibration can be identical across the model line, meaning that one could buy a base model car then get upgrade tires and wheels and still use the same speedo. > When it comes to aftermarket, I wish I could pin that question down for > you, but I can't. The answer depends on the actual rolling circumference [quoted text clipped - 12 lines] > Even the cops recognize that, which is partly why they give you 5mph > grace (or more depending on the jurisdiction) before whacking you. That's certainly true. The differences you are telling us about are there, maybe, but utterly insignificant UNLESS one tire is different than the rest. The small difference could be enough to trigger the Low Pressure Monitor if said monitor derives its information from the speed sensors. >> The OEM wheel, tire size, and speed rating will be closest to the >> manufacturer's intended speedometer calibration. [quoted text clipped - 10 lines] > that one could buy a base model car then get upgrade tires and wheels > and still use the same speedo. But they're all OEM, which is my point. SignatureTegger >>The OP wants a larger sidewall for some reason, not a smaller one. (I >>don't get the logic he's using, but it's not my decision.) [quoted text clipped - 6 lines] > After you guys work out all this technical minutia let me know what the > consensus is. The short answer is, no, there is no difference. The long asnwer is that the overall diameter has to be the same for the short answer to be true. For example, my truck has a 265/75x16. I could go to a 265/35x20 and the tire should be the same overall diameter, therefore no affect on the speedo. >>>The OP wants a larger sidewall for some reason, not a smaller one. (I >>>don't get the logic he's using, but it's not my decision.) [quoted text clipped - 14 lines] > could go to a 265/35x20 and the tire should be the same overall > diameter, therefore no affect on the speedo. Supposing the two sizes have identical unloaded diameters, the 35 will have the larger rolling circumference and the slower speedometer reading. SignatureTegger >>>>The OP wants a larger sidewall for some reason, not a smaller one. (I >>>>don't get the logic he's using, but it's not my decision.) [quoted text clipped - 18 lines] > have > the larger rolling circumference and the slower speedometer reading. NOT true. Well, not entirely true. For every inch increase in the rim, the aspect ratio is decreased 5%. The result is a overall diameter, or radius, or circumference (depending on the specy you prefer) will be virtually identical. The difference -- if done on all four corners -- is insignificant. There will be a small change to the speedometer -- a change measured in single digit percentages. The speedometer would be more sensitive to the circumference, but since circumference is calculated by diameter X pi, or radius X 2 X pi, then all are the same. I assume proper inflation, and ignore any argument that assumes improper inflation. Improper inflation causes a variance, and the variance is measured by the speed sensors, not the speedometer. >>>>>The OP wants a larger sidewall for some reason, not a smaller one. >>>>>(I don't get the logic he's using, but it's not my decision.) [quoted text clipped - 27 lines] > will be a small change to the speedometer -- a change measured in > single digit percentages. As I said all along. The point is that it is unlikely that there will be NO change. Another example for illustration: Assume a tire with 23.21" diameter (my 195/60-14). The sidewall of that tire measures 4.6". The front tires drop about .875" at the contact patch. That's about 19%. Now assume a tire of the same diameter, but a hypothetical size of 195/35-18. This gives the sidewall a height of 2.7". Now imagine that this same tire has a drop of .875" at the contact patch. That's /one-third/ of the sidewall height of only 2.7"! NO 35-series tire drops 7/8" at the contact patch. It cannot. Any owner of that tire would be convinced his tire was flat! And this means the effective rolling circumference of such a tire would be /larger/ than the original 195/60-14, giving a slower speedometer reading.. > The speedometer would be more sensitive to the circumference, but > since circumference is calculated by diameter X pi, or radius X 2 X [quoted text clipped - 3 lines] > improper inflation. Improper inflation causes a variance, and the > variance is measured by the speed sensors, not the speedometer. If the wheel is turning at a speed other than what the speedometer is expecting, the speedometer will reflect that. Period. It may not be enough to notice in everyday driving, but it will be reflected. ABS-based speed sensors only track DIFFERENCES between wheels, not absolute speeds. And if a lower pressure makes a difference to the ABS, it will also make a difference to the speedometer, provided the lower pressure tire is on the wheel that's on the "solid" side of the diff. SignatureTegger >>>>>>The OP wants a larger sidewall for some reason, not a smaller one. >>>>>>(I don't get the logic he's using, but it's not my decision.) [quoted text clipped - 63 lines] > it will also make a difference to the speedometer, provided the lower > pressure tire is on the wheel that's on the "solid" side of the diff. I don't see that it matters if all tires are the same equivelent of the original specification, or that they are all the same non-equivelent -- physical fitment issues aside. ABS does not look at different speeds of the tires, it looks at some tires turning and some tires not turning, it makes the not-turning tires turn. The speedo would not care about one tire turning at a different speed, ever. But none of this is in the realm of answering the OP's question. His question was not framed very well, but that's another discussion all by itself. In the example of the 195/60x14 and the 195/35x18, the sidewall goes from 4.6 to 2.7 inches, but the tire diameter goes from 23.2 to 23.4, and the tire circumference goes from 72.9 to 73.4, and the revolutions per mile go from 869 to 863. It's possible that in such a small sidewall that the pressure monitor might not work very well, or at all, if it's based on feed from the speed sensors. This might be an undesirable affect. But the speedo would be unafftected for all practical purposes. Since the new tire is miniscually larger, then the speedo would slow a tiny amount -- a current reading of 70 might be an actual speed of 66 and the new reading of 70 might be an actual speed of 68, which is a desirable change as opposed to an undesirable one. > None of that matters to the ABS if all tires on the ground are the > same size, even if the tires are not the size that is specified for > the car. The ABS/Low Tire Pressure systems look at differences in the > rotational speed of one tire vs. the others. That's right! And it can ONLY do that if the rolling circumference is DIFFERENT between the two wheels being monitored. If the wheels are traveling the same path on the same car but their rotational speeds are different, then one of them is presenting a smaller circumference to the road and has to spin faster to cover the same ground as the tire with the larger circumference. And if both wheel/tire assemblies being monitored are identical except for their tire pressure, then their rolling radii MUST be different, otherwise they'd spin at the same rotational speed. The lower-pressure tire's sidewalls create the smaller circumference by compressing more, which is what causes the smaller circumference. Back to the original subject: A taller sidewall (i.e.: 80) will distort more under load than a short one (i.e.: 45), and will cause a larger reduction in rolling radius from unloaded to loaded. It cannot be any other way. SignatureTegger > Is there any difference in speedometer/odometer performance between > wheels that have the same overall circumference but one has thin > sidewalls and the other has wide sidewalls? I know there is a handling > performance and comfort difference. I've always preferred the looks of > the extra wide sidewalls with small wheel hubs to the more popular huge > wheels with thin tires. Nope. If the overall diameter/circumference is the same, the speedo will be on the mark. Width does not play into it. > Is there any difference in speedometer/odometer performance between > wheels that have the same overall circumference but one has thin > sidewalls and the other has wide sidewalls? I know there is a handling > performance and comfort difference. I've always preferred the looks of > the extra wide sidewalls with small wheel hubs to the more popular huge > wheels with thin tires. Yes, maybe, no depends on the change in radius. Tire Rack and discounttiresdirect.com have alternate wheel/tire packages you can order and they are supposed to fit the car and speedometer error should be minimal. My dog loves to pee on those shiny bright new wheels. > Is there any difference in speedometer/odometer performance between > wheels that have the same overall circumference but one has thin > sidewalls and the other has wide sidewalls? I know there is a handling > performance and comfort difference. I've always preferred the looks of > the extra wide sidewalls with small wheel hubs to the more popular huge > wheels with thin tires. If done properly, the difference is very minor and amounts to a rounding (no pun intended) error in the range of a few percentage points. Depending on the goal, the difference can amount to correcting the natural error of the speedometer to requiring entirely new gears sets to bring the operating envelope of the engine back to a useful range. Jeeps and trucks like to put oversize tires on that can alter the overall gear ratio of the drive train. The cure for the problems this causes is to replace the gear set (ring and pinion gears) with a new set of a different ratio. Stock gears might be 3.55:1, for example, and the new tires require a change to 4.10:1 to bring the torque and horsepower curves back to a useful range. Since we're talking about Toyotas, and presumably passenger cars, it would be difficult to put tires on that require a change of gears. I wrote a calculator that tells me the size of a given tire, and I can then plug in other tire specs to see what the size of an alternative might be. Generally, I use it to determine the specs that would be needed when the rims are changed from 15 to 17, for example. As a general rule, if you have a 225/55x15 and want to see what happens with a 17" rim, you would need a 225/45. My calculator says that for every inch change in the size of the rim you would need a 5% change in the aspect ratio -- the middle number of 225 / 55 x 15. If you wanted to make the second number larger but keep the same rim diameter, then you'd have to go smaller on the first number. In the example tire spec that I used, 225 is the width of the tread, 55 is the height of the sidewall as a percentage of the tread width, and 15 is the diameter of the rim. 225 is the width in millimeters of the cross section of the tire (the width of the tread, more or less) and 55% of that is the height of the sidewall.In this example, the sidewall height is 123.75mm, or about 4.87 inches. > Since we're talking about Toyotas, and presumably passenger cars, it would > be difficult to put tires on that require a change of gears. I wrote a > calculator that tells me the size of a given tire, and I can then plug in > other tire specs to see what the size of an alternative might be. Hey! I did too! But I lost it. It's on a disk around here...somewhere... However, there is always one available here: http://www.miata.net/garage/tirecalc.html |
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