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Car Forum / Jeep / January 2007Tires and speedometer
I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel wheels and tires. Does my speedometer need to be adjusted? Has anyone done this? Tom Jeep speedometers generally read slow stock, you are probably closer to the correct reading now than before. (but still slow) >I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel > wheels and tires. Does my speedometer need to be adjusted? > > Has anyone done this? > > Tom Larger tires will make the speedo even slower than it was before. The truth is, most speedos read fast from the factory. The idea is that if the speedo says 60, and the speed limit is 60, then you do not want to be exceeding 60, therefore the actual speed is more like 57. The speedo reads fast. New tires will make the speedo read slow, therefore the ground speed of 57 will be displayed closer to 57 than before, the speedo will read slower. I am of the opinion that the change he made will be greater than the factory error, and the new 60 will really be something like 63. The speedo will read too slow because the actual ground speed will be greater than the indicated speed. The potential problems with this should be obvious. > Jeep speedometers generally read slow stock, you are probably closer to > the correct reading now than before. (but still slow) [quoted text clipped - 5 lines] >> >> Tom You need to find a measured mile or use a GPS to see what it is now. I have found that Jeep speedometers are calibrated for the largest stock tire size so yours might be right on now. Our XJ's became right on with 235's which are almost 30" tall. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel > wheels and tires. Does my speedometer need to be adjusted? > > Has anyone done this? > > Tom I agree. Sorta. If the factory fitment of speedo gear was to accomodate the largest factory tire, then with the small tires on the stock models, the speedo should be fast, they should read considerably higher than the actual speed. If the speedo gear for the stock base-models is selected properly for the tires, then changing the tires should result in an error where the speedo reads slow with the new tires. Your experience with the XJ was such that you swapped a tire that was nearly 30 with one that was 30, and the error that caused dialed the speedometer's accuracy in almost perfectly. (I had a similar experience with my BMW, by the way.) But, the OP is making a drastic change from a 26.5" tire to a 30. This is 3" different in diameter, and 9" in circumference. That's a large difference, and the speedo should logically not be able to display the actual speed anymore. > You need to find a measured mile or use a GPS to see what it is now. I > have found that Jeep speedometers are calibrated for the largest stock [quoted text clipped - 15 lines] >> >> Tom The funniest one was my CJ7's speedometer. It became proper with 33" tires for some reason. It had 31's on it when I got it and it was slow to the real speed. I would be doing a real 60 when the thing read 65. I think someone swapped a gear to go to 31's from the 215's when 30's would have been right for the speedometer... Mike > I agree. Sorta. > [quoted text clipped - 33 lines] >>> >>> Tom My TJs one came right with 31s. Dave > The funniest one was my CJ7's speedometer. It became proper with 33" > tires for some reason. It had 31's on it when I got it and it was slow [quoted text clipped - 42 lines] > >>> > >>> Tom I think your speedo will read SLOW now. When it reads 60, yoiur actual speed will be 65, for example. (You'll need to take measurements to arrive at the actual error.) Your original tires were about 26.5 inches in diameter, the new tires will be closer to 29.5. This gives a diameter increase of about 3 inches, which will give a circumference increase of around 9 inches. You will need to calibrate the speedo as a result. It ought to be as easy as swapping a gear on the speed sensor in the transmission, or wherever they mounted the sensor on your truck. >I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel > wheels and tires. Does my speedometer need to be adjusted? > > Has anyone done this? > > Tom Speedometer ratio adjustment calculation. Ever wonder how far off your speedometer is with your new bigger tires? By using a simple ratio calculation, this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assumimg nothing but tire size has changed). If you know specific information about your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator, it's probably a little more accurate. Formula used (New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH Another way of looking at this relationship would be to figure what the indicated speed would be if you were actually going 60 mph. In this case, the tire diameter relationship is flip-flopped to: Indicated Speed = old tire diameter x actual speed New tire diameter Using the previous example, your speedometer reading at an actual 60 mph is: 28 x 60 = 48 mph 35 http://www.offroaders.com/info/tech-corner/reading/mph-rpm.htm Here's the site. >I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel > wheels and tires. Does my speedometer need to be adjusted? > > Has anyone done this? > > Tom Easy way to find out, beg borrow or steal one of these GPS navigation devices, they give your speed and are pretty much dead on. There are mile markers along every highway. God Bless America, Bill 0|||||||0 mailto:LWHughes3rd@aol.com http://www.billhughes.com/ > Easy way to find out, beg borrow or steal one of these GPS navigation > devices, they give your speed and are pretty much dead on. |
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