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john hamilton · 06-06-2011, 08:57 PM

It's just steps, not hard if you pay attention:

(tire dia. in inches X 3.1416)/ 12" per foot = 28 X 3.1416/12 = 7.33' traveled per revolution

5280' per mile/feet traveled per revolution = 5280/7.33 = 720 revolutions per mile

revs of the tire per mile X gear ratio = 720 x 7.33 = 3197 engine revs per mile so at 60mph you would be wrapped out.

correct for 40mph which is less than a mile per minute by multiplying engine revs per mile X actual mph/60 = 3197 x 40/60 = 2132 engine rpms at 40 mph.

With your tires and gears you can apply this correction to any speed by putting your "actual mph" in the equation above.

06-06-2011, 08:57 PM	#7
john hamilton Senior Member Join Date: May 2010 Location: Waxahachie, Texas Posts: 241	Re: rpm math question It's just steps, not hard if you pay attention: (tire dia. in inches X 3.1416)/ 12" per foot = 28 X 3.1416/12 = 7.33' traveled per revolution 5280' per mile/feet traveled per revolution = 5280/7.33 = 720 revolutions per mile revs of the tire per mile X gear ratio = 720 x 7.33 = 3197 engine revs per mile so at 60mph you would be wrapped out. correct for 40mph which is less than a mile per minute by multiplying engine revs per mile X actual mph/60 = 3197 x 40/60 = 2132 engine rpms at 40 mph. With your tires and gears you can apply this correction to any speed by putting your "actual mph" in the equation above.