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Car Forum / Ford / Ford Trucks / November 2006Changing 18" Rims for 20"
Does anyone know how to calculate the difference in rear end ratio with the difference in rim size? TIA > Does anyone know how to calculate the difference in rear end ratio with the > difference in rim size? > > TIA If you do what is called zero stepping there is no change. You move up in rim size but pick the tire size to maintain the same overall diameter. Whitelightning > Does anyone know how to calculate the difference in rear end ratio with the > difference in rim size? > > TIA It doesn't matter what the rim size is. it is the final tire diameter that will change your final ration. If you have a 31" tire on 18" rims and put a 31" tire on 20" rims there is no change. >> Does anyone know how to calculate the difference in rear end ratio with >> the difference in rim size? [quoted text clipped - 3 lines] > will change your final ration. If you have a 31" tire on 18" rims and put > a 31" tire on 20" rims there is no change. Not quite. The tire/wheel combination will not change the differential ratio (the gears inside do that) but what the op is asking is how to calculate the effects of different sized wheel/tire combo on (I assume) the speedo and the engine performance. The formula is thus: tire width X sidewall X 2 + rim diameter. For example; a 235/75/15. 235 is the tire width in millimeters, 75 is the sidewall height expressed as a per cent of the tire width, and 15 is the wheel diameter in inches. OK....235 X .75 = 176.25mm. (sidewall height in millimeters). However, the tire diameter is in inches so convert the sidewall figure to inches by - sidewall height/25.4 (IIRC) = 6.94in. Now multiply that by 2 (because there is an upper and lower sidewall) - 6.94 X 2 = 13.87in. Lastly, add in the wheel diameter: 13.87 + 15in = 28.87in. This is the wheel/tire combo diameter. If the op calculates the wheel/tire diameter for both his current set up and for the proposed set up, he will have an idea of the difference. Of course, to get an even better perspective he should multiply the wheel/tire diameter by pi to get the circumference of the tire which will tell him how far the tire will travel in one rotation. The goal in changing wheel size is to try to stay as close as possible to the size of the wheel/tire combo that was on the vehicle from the manufacturer. For the op (Van Dyke) Hope this helps you get the info you desire. Dave D >>> Does anyone know how to calculate the difference in rear end ratio with >>> the difference in rim size? [quoted text clipped - 26 lines] > > Dave D I know what the numbers mean, but if the final diameter of the tire/wheel combo is 31" there is no change to his final ratio. 31" diameter is 31" diameter. >>>> Does anyone know how to calculate the difference in rear end ratio with >>>> the difference in rim size? ///snipped/// > I know what the numbers mean, but if the final diameter of the tire/wheel > combo is 31" there is no change to his final ratio. 31" diameter is 31" > diameter. For sure. I didn't mean to contradict your statement but the op asked how to calculate the difference if he changed wheel. That is what I was attempting to provide him. DaveD >>> Does anyone know how to calculate the difference in rear end ratio with >>> the difference in rim size? [quoted text clipped - 7 lines] > ratio (the gears inside do that) but what the op is asking is how to > calculate the effects of different sized wheel/tire combo Yes, and so far everybody has answered him correctly. The TIRE SIZE is what it takes. >>>> Does anyone know how to calculate the difference in rear end ratio with >>>> the difference in rim size? [quoted text clipped - 10 lines] > Yes, and so far everybody has answered him correctly. The TIRE SIZE is > what it takes. Not entirely. It is the size of the wheel tire combination that makes the difference. Furthermore, his original question was how to caluculate the difference. That is what I gave him. DaveD >>>>> Does anyone know how to calculate the difference in rear end ratio >>>>> with the difference in rim size? [quoted text clipped - 16 lines] > > DaveD No Dave. Well, you are partly right. But, if the OP has 31s now, and has 31s when he is finished spending money, then the change to the speedo and engine performance will be zero. No change. Nothing. If he actually uses your calculator and plugs in the right variables to buy a tire with the same overall diameter (larger rim size but smaller sidewall area), then he will see no change in anything. Since he hasn't been back since he asked his question, my guess is he simply went to the tire store and they answered his question for him. >>>>>> Does anyone know how to calculate the difference in rear end ratio >>>>>> with the difference in rim size? [quoted text clipped - 29 lines] > Since he hasn't been back since he asked his question, my guess is he > simply went to the tire store and they answered his question for him. Even with no change in overall diameter, there can still be a loss of acceleration due to the larger rim's greater moment of inertia. >>>>>>> Does anyone know how to calculate the difference in rear end ratio >>>>>>> with the difference in rim size? [quoted text clipped - 28 lines] > Even with no change in overall diameter, there can still be a loss of > acceleration due to the larger rim's greater moment of inertia. That may be, but the difference in weight will not make that muh difference. Also the original question was for what is the difference in rear end ration. >>>>>>> Does anyone know how to calculate the difference in rear end ratio >>>>>>> with the difference in rim size? [quoted text clipped - 32 lines] > Even with no change in overall diameter, there can still be a loss of > acceleration due to the larger rim's greater moment of inertia. I suspect his truck has more than enough torque to make the tires go around. >>> Does anyone know how to calculate the difference in rear end ratio with >>> the difference in rim size? [quoted text clipped - 8 lines] > calculate the effects of different sized wheel/tire combo on (I assume) > the speedo and the engine performance. That's the point. If one makes the right choice in tires, changing from a 31" tire mounted on a 15" rim to a 31" tire mounted on a 20" rim, the difference to the speedo will be zero. The change to engien performance is also zero. The formula is thus: tire width X sidewall X 2 + rim > diameter. For example; a 235/75/15. 235 is the tire width in millimeters, > 75 is the sidewall height expressed as a per cent of the tire width, and [quoted text clipped - 11 lines] > as close as possible to the size of the wheel/tire combo that was on the > vehicle from the manufacturer. And, when one does this (buys the right combination of tire and wheel), there is no change to the speedo or the gearing or the engine performance. The easiest way to accomplish the correct tire/wheel purchase is to ask the tire store. Try tirerack.com or tires.com for calculators and pricing of the various options. Rims don't make any difference at all. It's the tire size that we use for that. > Does anyone know how to calculate the difference in rear end ratio with > the difference in rim size? > > TIA "William H. Van Dyke" wrote: Does anyone know how to calculate the difference in rear end ratio with the difference in rim size? ________________________________________________ Changing the tire diameter (not the rim diameter) is equivalent to changing the rear end ratio instead, according to a formula: (EQUIV REAR RATIO) = (ACTUAL REAR RATIO) X (STOCK TIRE DIA) / (NEW TIRE DIA) EXAMPLE: If (ACTUAL REAR RATIO) = 3.73 If (STOCK TIRE DIA) = 30 If (NEW TIRE DIA) = 32 Then (EQUIV REAR RATIO) = (3.73) X (30) / (32) = 3.50 If it is the speed ratio you need it is just the tire diameter ratio: (SPEED RATIO) = (NEW TIRE DIA) / (STOCK TIRE DIA) EXAMPLE: If (STOCK TIRE DIA) = 30 If (NEW TIRE DIA) = 32 Then (SPEED RATIO) = (32) / (30) = 1.067 So 60 mph on the stock tires would become 1.067 X 60 = 64.02 mph on the new tires at the same engine rpm. HTH Rodan. The change in rim size may not reguire a change in gear ratio. 265 70 16 30.606 275 60 18 30.992 As you can see, these two tires are so close to being the same size, the gear ratio would not change, and the speedometer would remain pretty nearly the same as it is now. The reality is that the 18s would likely make the speedometer dead-nuts accurate instead of having the built in error it has today. > Does anyone know how to calculate the difference in rear end ratio with > the difference in rim size? > > TIA OOPS. I compared a 16 and 18. The same variance holds true for the comparison between 18s and 20s, except the diameter is 32 and a fraction insgtead of 30 and a fraction. (The fraction remains about the same, and the difference in the fractions is about the same.) Your tire store will know precisely what 20" size will give the same overall diameter as the current 18s that you have today. > The change in rim size may not reguire a change in gear ratio. > [quoted text clipped - 18 lines] >> >> TIA |
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