Question: how many rpms are in a mph?
Answer: Jeff Greene, Sport Car enthusiast. Original question: What's the formula to convert rpm to mph and mph to rpm? If you have a tachometer and a speedometer, you divide the rpm by the speed, so 50 miles per hour or 80 kilometers per hour at 2000 rpm would be 40 rpm per mile per hour or 25 rpm per kilometer per hour.