Calculating engine rpm at speed...
Just a little basic algebra here...
Pi times tire diameter gives tire circumference. (3.1416 * D)
5280 feet per mile times 12 gives inches per mile. (5280 * 12) = 63360 inches per mile.
Therefore 63360 / (3.1416 * D) = tire revolutions per mile.
60mph = 1mile per minute.
Therefore 60mph = 63360 / (3.1416 * D) = tire revolutions per minute.
Are you following me so far? Good.
A P215/70/14 tire is about 26.5" tall.
Plug that value in for D and run the formula.
63360 / (3.1416 * 26.5) = 63360 / 83.25 = 761 tire revolutions per mile.
That means that at 60mph, this tire is rotating 761 times per minute.
now we throw gear ratio into the equation...
To determine driveshaft speed, you must multiply by the rear end ratio.
So, at 60mph, with a 26.5" tire, your driveshaft RPM values would be:
761 * 2.75 = 2093
761 * 3.00 = 2283
761 * 3.25 = 2473
761 * 3.50 = 2663
761 * 3.70 = 2815
761 * 3.91 = 2975
761 * 4.11 = 3128
With most three speed automatics or conventional stick shift transmissions the ratio of engine rpm to driveshaft rpm is 1:1. That means you can stop now. The driveshaft speed equals your engine speed.
If you're running an overdrive, that means the engine is turning slower than driveshaft rpm, so that correction factor can be thrown in now as well.
I
think that an AOD automatic has a ratio of .78, so multiply that driveshaft rpm by the AOD ratio to get the engine speeds. (Hey, what do I know about these new-fangled things... I'm a
Senior Motorhead!

)
So, with a 26.5" tire and 3.70 rear gears and an AOD transmission:
761 * 3.70 * .78 = 2196 rpm at 60 mph.
I hope this math refresher helps all you Cats!