I initially set the unit for 34" tires, the diameter of my Toyo ATIIs in 295 60r20, the speedometer was actually a couple mph over. After some downward-tweaking for tire inflation level, I setting the unit to correct for 32.75 inch tires got my speedometer accurate to within less than 0.5mph error compared to Garmin GPS. Keep in mind that actual tire diameter is dependent on how many psi you are running, so getting things right on the money may take some tweaking.
The unit did not work for me
After installation The vehicle could barely start and then it stalled out and would not restart . I called jet and they seem to have a system down on how to deal with customers who have problems with their products . So it cost me 15 dollars to ship it back to CA so jet can tell me if it works or not and what they will do about it. I mean the chip doesn't even fit right into the computer.