Then multiply both of these individual products together, and divide by the overall gear ratio times the number of inches in a mile (63,360). "336" is simply the reduced, rounded-off whole-number ...

If we do the math in reverse and divide 87 (the new tire size) by our FDR number, we get a new gear ratio of 5.74. That is what our rear gear should be in order to keep our FDR at 15.1663.

This is a simple two-step process. First, figure out how fast the rear wheels will turn by dividing the SR's 7500-rpm redline by first the gear ratio, and then the final drive ratio.

For example, if a 3.73 axle ratio currently is matched with a 31.2-in.-diameter tire, then the tire is decreased to 30.2 in., the revolutions per mile increase from 650 to roughly 670.

One of those tools is the American Expedition Vehicle (AEV) ProCal Module. The ProCal allows easy calibration of gear ratio, tire size, the tire pressure threshold of the TPMS, and other useful ...