What is the number of revolutions per minute of a car going 50 mph and tire diameter of 2.5 ft?

560 rpm The car is not revolving. If you want to know the engine rpm you would need to know the ratio of the differential (final drive) and the ratio of the gear that you are in. If you are asking for the number of revolutions per minute for the tire, you would calculate it as follows. Tire circumference is: 3.141 x 2.5 ft = 7.85 ft A revolution of the wheel would be 7.85 ft/rev so the reciprocal gives: a revolution is 0.1274 rev/ft. 50 mi/hr x 0.01667 hr/min x 5280 ft/mi = 4400 feet/min 0.1274 rev/ft x 4400 ft/min = 561 rev/min (or rpm) Note: Previously, the answer was rounded to 3 significant digits. If you consider that 50 mph is precise to the units place you can really only support two significant digits in the final answer so it needs to be rounded to 560 rpm.