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I checked my truck today. 1999 Silverado. The speedo reads the correct speed but the tire size is wrong when I checked the files. How would I go about changing it to the correct tire size. I'll be changing the size soon to a bigger tire and want it to all read right. Right now it says it has 33.2" tire but it actually has 35" tires. Not sure how to correct it
Tire diameter in the calibration never matches the actual tire diameter. It is always smaller. It is usually around .8" smaller than actual. And just because your tire is sized 35" doesn't mean it is actually 35" diameter. Lots of 35" tires can have an inch of tread depth that with wear effects actual diameter. What is more important is what the scanner shows for MPH matches what a GPS shows for MPH. If the scanner now shows correct MPH then take the difference in diameter between the tire on it now and the new tire and add that to the size the calibration shows. I've always suspected that the speedometer is actually factory calibrated by ppm settings and the software is using that to calculate and display a tire diameter.