Engine RPM Calculator
Enter your speed, rear axle ratio, transmission gear ratio and tire diameter to see what RPM the engine turns, such as your cruising RPM on the highway. It can also solve for gear ratio, tire diameter or speed.
How to calculate engine RPM
RPM = MPH × axle ratio × transmission ratio × 336 ÷ tire diameter
Tire diameter is in inches. The 336 turns miles per hour into tire revolutions per minute: a mile is 63,360 inches and an hour is 60 minutes, so 63,360 ÷ (60 × π) = 336.1. Dividing by the tire diameter gives how many times the tire turns each minute, and the axle and transmission ratios multiply that up to engine speed.
The formula assumes the drivetrain has no slip. That holds for a manual transmission or an automatic with the torque converter locked up. With the converter unlocked, the engine turns a little faster than the formula shows.
Example: cruising RPM at 65 mph
A truck on 265/70R17 tires (31.6 inches tall, from the tire size calculator) with 3.73 gears, cruising at 65 mph in a 0.70 overdrive:
65 × 3.73 × 0.70 × 336 ÷ 31.6 = 1,805 RPM
In a 1.00 direct-drive top gear, the same truck turns 2,578 RPM at 65 mph.
Highway RPM at 70 mph with a 0.70 overdrive
| Axle ratio | 26" tire | 28" tire | 30" tire | 32" tire | 33" tire | 35" tire |
|---|---|---|---|---|---|---|
| 3.08 | 1,950 | 1,811 | 1,690 | 1,585 | 1,537 | 1,449 |
| 3.42 | 2,166 | 2,011 | 1,877 | 1,760 | 1,706 | 1,609 |
| 3.73 | 2,362 | 2,193 | 2,047 | 1,919 | 1,861 | 1,755 |
| 4.10 | 2,596 | 2,411 | 2,250 | 2,109 | 2,046 | 1,929 |
| 4.56 | 2,888 | 2,681 | 2,503 | 2,346 | 2,275 | 2,145 |
Highway RPM at 70 mph in direct drive (1.00)
| Axle ratio | 26" tire | 28" tire | 30" tire | 32" tire | 33" tire | 35" tire |
|---|---|---|---|---|---|---|
| 3.08 | 2,786 | 2,587 | 2,415 | 2,264 | 2,195 | 2,070 |
| 3.42 | 3,094 | 2,873 | 2,681 | 2,514 | 2,438 | 2,298 |
| 3.73 | 3,374 | 3,133 | 2,924 | 2,742 | 2,658 | 2,507 |
| 4.10 | 3,709 | 3,444 | 3,214 | 3,014 | 2,922 | 2,755 |
| 4.56 | 4,125 | 3,830 | 3,575 | 3,352 | 3,250 | 3,064 |
Calculated with the formula above, assuming no converter slip.
How tires and gears change RPM
RPM is inversely proportional to tire diameter. Moving that 3.73-geared truck from 31.6-inch to 33-inch tires makes the tire 4.4% taller and drops its RPM at 70 mph from 1,943 to 1,861, about 4.2% lower. A numerically higher axle ratio does the opposite and raises RPM in direct proportion.
If your RPM is off after a tire swap, the gear ratio calculator finds the axle ratio that brings it back. To go the other way and find your speed from RPM, or your top speed in each gear, use the vehicle speed calculator.
Frequently Asked Questions
How do you calculate engine RPM from speed?
Multiply your speed in mph by the axle ratio, the transmission gear ratio and 336, then divide by the tire diameter in inches.
Where does the 336 in the RPM formula come from?
It converts miles per hour into tire revolutions per minute. There are 63,360 inches in a mile and 60 minutes in an hour, and 63,360 divided by 60 times pi is 336.1.
What RPM will I cruise at on the highway?
It depends on your gears, tires and overdrive. A truck with 3.73 gears, a 0.70 overdrive and 31.6-inch tires turns about 1,943 RPM at 70 mph.
How do bigger tires change RPM?
RPM is inversely proportional to tire diameter. Going from a 31.6-inch to a 33-inch tire makes the tire 4.4% taller and lowers RPM by about 4.2%.
What transmission ratio do I use without overdrive?
Use 1.00 for a direct-drive top gear. With a 0.70 overdrive, the engine turns 70% of the RPM it would in direct drive at the same speed.
Does the torque converter change the RPM?
Yes. The formula assumes no slip, which matches a manual transmission or a locked torque converter. With the converter unlocked, the engine runs somewhat faster than the calculated RPM.
