Originally posted by tomfiii
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60 mph equals a mile a minute, or 5280 feet per minute. 5280 feet divided by tire circumference gives you rear wheel rpm (revolutions per minute).
Now divide # of teeth on rear wheel by # of teeth on tranny output (should be around 1.9 or so) and multiply times wheel rpm to get tranny output rpm at 60 mph.
Now get the ratio of the ring and pinion by dividing the # of teeth on the ring gear by the # of teeth on the pinion gear and multiply that ratio times the tranny output rpm. That final result will be your engine rpm at 60 mph. .. To get your rpm at 45 mph multiply your 60mph result times (45/60 = .75)
Rpm at 75 (75/60) =1.25 x your 60mph result.
use a calculator, please .. can do long hand with a little rounding, but....
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