Or from another post:
"The circumference is pi x D or: 3.141593 x 35 = 109.955755 inches.
You have to convert that to millimeters by multiplying by 25.4, which gives an unloaded circumference of 2792.88mm.
But now, your heavy truck flattens the tire at the ground which reduces its effective diameter/circumference. Usually, this reduction is about 3%, so multiply the last figure by 0.97 and you get 2709mm (rounded to the nearest whole number).
Try that last number. If the Gryphon's speed is slower than the real speed, increase it. If the Gryphon's speed is faster than the real speed, decrease it. Your dash speedo will always read about 2-3mph faster than the Gryphon, so your goal is to get the Gryphon speed correct (the same as a GPS). When you get the Gryphon's speed right, the dashboard odometer will be correct."