How engine RPM relates to speed
For every turn of the wheels, the engine turns a fixed number of times set by the transmission gear and the axle (differential) ratio. Multiply those two ratios together to get the overall drive ratio. The tire's size then decides how far one wheel turn moves the car.
RPM formulas
mph = RPM × tire diameter ÷ (axle ratio × transmission ratio × 336)
336 ≈ 63,360 in per mile ÷ 60 min ÷ π
Worked examples
- Highway cruise: 31.6 in tires, 3.73 axle, 0.67 overdrive, 70 mph: 70 × 3.73 × 0.67 × 336 ÷ 31.6 = 1,860 rpm.
- Speed at a given RPM: the same truck in a 1.00 gear at 3,000 rpm is doing about 75.6 mph.
Typical transmission ratios
| Gear | Typical ratio range |
|---|---|
| 1st | 3.0 to 4.7 |
| Middle gears | 1.3 to 2.2 |
| Direct drive | 1.00 |
| Overdrive | 0.60 to 0.85 |
Your exact ratios are in the owner's manual or a transmission spec sheet. Need your tire height? Use the tire diameter calculator. Changing gears for bigger tires? Try the gear ratio calculator.
Frequently asked questions
How do you calculate engine RPM from speed?
Multiply the speed in mph by the axle ratio, the transmission gear ratio and 336, then divide by the tire diameter in inches. 70 mph with a 3.73 axle, 0.67 overdrive and 31.6 inch tires is about 1,860 rpm.
What RPM should I cruise at on the highway?
Most modern cars and trucks cruise between about 1,500 and 2,500 rpm at 65 to 75 mph in top gear, which keeps fuel consumption and noise low.
Do bigger tires lower RPM?
Yes. A taller tire covers more distance per turn, so the engine turns fewer times at the same speed, which can make the car feel sluggish until the gearing is changed.
Where does the number 336 come from?
It converts miles per hour and tire diameter in inches into revolutions per minute: 63,360 inches in a mile divided by 60 minutes and by pi is about 336.
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Last updated October 2026.