text: convert rpm to mph?   you need to know the gear ratio in the transmission and the final drive ratio.  here is an example:  32" diameter tire spins 32*3.14159=100.5" per turn, or 8.38 feet per turn.  say the engine is spinning at 2100 rpm in overdrive.  overdrive on my car is 0.74 (as showin in specifications for transmission.)  my final drive ratio is 3.73:1.  so every time the propeller shaft turns 3.73 times, the tire spins one time.  2100 engine rpm * 1/0.74 * 1/3.73=760 wheel rpm.  so the engine speed is first increased by the overdrive, then reduced by the final drive.  the wheel spinning at 760 rpm will give 8.38 feet * 760 = 6369 feet per minute. multiply by 60 to get 382128 feet per hour.  now, in one mile there are 5280 feet.  so 382128 feet/hr * 1/5280 feet/mile = 72 mph.  note: numbers have been rounded off.  ooops--for a bicycle wheel do the following: take the diameter of the wheel, multiply by pi.   26" wheel * 3.14159/12 = 6.8 feet per turn.  if the wheel is spinning at 380 rpm, then at 6.8feet/turn = 2584 feet per minute * 60 = 155040 feet per hour *1/5280 feet/mile = 29 mph.