I pulled this from another board that TEX posted it on.
"I frequently do math to try to make predictions about which gear ratios will work. For example,
FD*MPH*TD/336 = RPM
FD = Final Drive (which equals tranny gear*t-case gear*axle ratio)
MPH = Miles Per Hour
TD = Tire Diameter
RPM = engine Revolutions Per Minute
You can also turn the ratio around to say that:
MPH = (RPM*TD)/(336*FD)
Using that ratio, you can calculate the effect of different tires & gear ratios from a baseline.
Let's say that most Auto trannies have a 2.5:1 first gear, 1.5:1 second, & 1:1 3rd (not taking into account the effect of converters). And most transfer cases are 1:1 in high range & 2:1 in low.
And let's say you have a truck that works well in low-range, 2nd gear, with 4.10 gears & 35" tires. And you want to know what gears you'd need to run 1st gear high range with everything else on the truck staying the same. Well, you'd plug all of the numbers into the formula using your max RPM to determine tire speed. Then change the t-case ratio & tranny ratio & recalculate with the same RPM & MPH to find your "new" gear ratio."
what kind of track are you running? deep mud or shallow fast track? Tex will be on here before long with more info for you but this should get you started.