Engine speed in revolutions per minute (RPM) is set by how fast you are going, how large your tires are, and the gearing between the engine and the wheels. Knowing that relationship is useful when you check how loud a car will be at highway speed, compare gear ratios before a swap, or work out what speed a gear can reach before the redline. This engine RPM calculator turns road speed, tire diameter, transmission gear ratio and final drive ratio into engine RPM, and optionally into the speed at which you would reach a chosen redline.
How to use the engine RPM calculator
- Enter the road speed and choose mph or km/h.
- Enter the overall tire diameter. You can read it from the sidewall markings with the tire diameter calculator; a 265/70R17 tire is about 31.6 inches tall.
- Enter the transmission gear ratio for the gear you are in. Use 1 for a direct-drive gear, and a number below 1 for an overdrive.
- Enter the final drive (axle) ratio, for example 3.73. Your owner's manual or the build sheet lists it.
- Optionally enter the redline to see the top speed in that gear.
Engine RPM formula
Engine RPM = wheel RPM × gear ratio × final drive ratio
Speed at redline = redline ÷ (gear ratio × final drive) ÷ 60 × π × tire diameter
In the common U.S. units this becomes RPM = mph × 336.13 × gear × final ÷ tire diameter in inches, because one mile is 63,360 inches and one hour is 60 minutes (63,360 ÷ 60 ÷ π ≈ 336.13). The calculator uses exact unit definitions: 1 mph = 0.44704 m/s and 1 km/h = 1/3.6 m/s. It assumes no wheel slip and no torque-converter slip; an automatic with a converter that is not locked will run a little higher.
Worked example
A car with 26-inch tires, a 3.73 final drive and an overdrive gear of 0.74 travels at 70 mph. The engine redlines at 6,500 rpm.
- Speed = 70 × 0.44704 = 31.29 m/s; tire diameter = 26 × 0.0254 = 0.6604 m.
- Wheel RPM = 31.29 ÷ (π × 0.6604) × 60 = 905.
- Engine RPM = 905 × 0.74 × 3.73 = 2,498.
- Speed at a 6,500 rpm redline in this gear = 182 mph.
Engine RPM by speed and gear ratio
26-inch tires and a 3.73 final drive. The columns show the transmission ratio of the gear you are in:
| Speed (mph) | Gear 1.00 | Gear 0.80 | Gear 0.70 | Gear 0.60 |
|---|---|---|---|---|
| 30 | 1,447 | 1,157 | 1,013 | 868 |
| 45 | 2,170 | 1,736 | 1,519 | 1,302 |
| 55 | 2,652 | 2,122 | 1,857 | 1,591 |
| 65 | 3,134 | 2,508 | 2,194 | 1,881 |
| 70 | 3,376 | 2,700 | 2,363 | 2,025 |
| 75 | 3,617 | 2,893 | 2,532 | 2,170 |
| 80 | 3,858 | 3,086 | 2,700 | 2,315 |
Engine RPM at 65 mph by tire size and axle ratio
In a 0.74 overdrive gear. Taller tires lower the RPM; a numerically higher axle ratio raises it.
| Tire diameter (in) | 3.08 | 3.42 | 3.73 | 4.10 |
|---|---|---|---|---|
| 24 | 2,075 | 2,304 | 2,513 | 2,762 |
| 26 | 1,915 | 2,127 | 2,320 | 2,550 |
| 28 | 1,778 | 1,975 | 2,154 | 2,367 |
| 30 | 1,660 | 1,843 | 2,010 | 2,210 |
| 32 | 1,556 | 1,728 | 1,885 | 2,072 |
Using the result
- Cruising noise and fuel use. Lower RPM at highway speed is usually quieter, though the best RPM for fuel economy depends on the engine.
- Changing tires. A taller tire lowers RPM at a given speed and makes the speedometer read low; see the speedometer error calculator.
- Changing gears. To compare axle ratios, use the gear ratio calculator.
- Staying under the redline. Use the optional redline box to find the speed limit of each gear.
Frequently asked questions
How do I calculate engine RPM from speed?
Divide speed by the tire circumference to get wheel RPM, then multiply by the transmission gear ratio and the final drive ratio. In mph and inches: RPM = mph × 336.13 × gear × final ÷ tire diameter.
What RPM should my engine run at 70 mph?
It depends on your tires and gearing. With 26-inch tires, a 3.73 axle and a 0.74 overdrive gear the answer is about 2,500 rpm. Enter your own ratios to see yours.
Do bigger tires lower RPM?
Yes. A taller tire covers more ground per turn, so the wheels and engine turn slower at the same road speed. It also makes the speedometer read low unless corrected.
Where do I find my gear ratios?
Check the owner's manual, a service manual or the manufacturer's specification sheet for the transmission gear ratios and the axle ratio. The axle ratio is also often on a tag on the differential.
Why is real RPM a little higher in an automatic?
A torque converter that is not locked slips slightly, so the engine turns faster than the pure gear math predicts. Most modern automatics lock up at cruise speed, where the result matches the calculator.