I am playing with my special forces software and I am having trouble with the speedometer. I have 315/75x16 tires, dia is 34.75. I check my speed with a gps and 34.5 is 4.3 mph slow, 35.5 is 2.5mph slow and 36.5 is 4.9 mph slow. I am confused. I was hoping to be closer to dead on with the 36.5 but it seems to me off more than 35.5.

Has anyone been able to calibrate thier speedos and verify it with either radar or gps?