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.
Great tool for the job
After getting rid of the performance robbing 20-inch wheels and tires and installing base model 245/70R17s, the speedo needed correction. Knowing the exact percentage of the change unfortunately doesn't help, so it's a bit of trial and error to get it right since tire height is the only usable criterion. With 30.6 inch tires, and no static loaded radius data, I guessed and used 30 inches. Very, very close. Tried with 30.25", but went back to 30, which is quick and easy with the Hypertech.