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 old gas hog!!!
I like the programmer, I wish it could do more to help or improve the gas mileage on my truck! 16 MPG before the installation, and 18 mpg after the installation.The increase was and is appreciated, I would like 24 or 25 MPG even better.I guess you can only do so much, with what you have to work with.