I replaced factory tires with the Firestone Destination ATs. I went to a 255/70R17. The spec from Firestone was 31.1. I bought the hypertech speedometer programmer as well. Here is what I found. I actually did the math (pie*d) and then measured against the GPS. If you install a tire my size, you will immediately be under by about 6% on the speedometer and mileage. It also appears to have some impact on how often the econ mode is used by the jeep. When I programmed it, it had the option of programming 31.0 or 31.25. I used 31.25 and went over by 4.5%. GPS confirmed. I reprogrammed to 31 and it was at approx 99.5% of the speed (very close). I know it is probably in my head, but the shifts, and feel of the engine seam better. My GPS and speedometer are dead on. Mileage should be accurate. I would bet that a local shop (maybe a dealer but certainly not mine) would program it for you cheaply if you didn't want to drop the 180 bucks for the hypertech.