Quote Originally Posted by techinspector1 View Post
It is the CIRCUMFERENCE of the tire that runs down the road, not the diameter or loaded radius. When you mount your new tires, air them to spec, then use a cloth or fiberglas tape to find the circumference of the tire, which you can then convert mathematically to the diameter to use in the formulas that you find online. D=C/pi.
100% correct on the circumference point, Tech. I always use the published diameter spec for the tire I'm running, which is a calculated value at recommended inflation as opposed to measured, but it's probably better to say:

Cruise RPM = (mph x Gear Ratio x 107 x OD Ratio)/Tire Circumference in inches (at normal inflation)

The "336" fudge factor that keeps the units right was divided by Pi - the Devil's in the details......