RPM Calculator by Gear Ratio and Tire Size
Use the Gearstar RPM calculator to find your engine RPM at any speed. Enter your MPH, tire diameter and rear gear ratio, then pick your transmission and gear. The calculator fills in the transmission gear ratio for you and returns engine RPM.
Engine RPM formula: RPM = (MPH × rear gear ratio × transmission gear ratio × 336) ÷ tire diameter in inches.
Results are a close estimate. They do not account for torque converter slip, tire growth at speed, drivetrain loss or road conditions.
How to Use the RPM Calculator
- Select your transmission model and gear. The transmission gear ratio fills in automatically. Pick the top gear (overdrive, if you have one) to see your highway cruising RPM.
- Enter your speed in MPH. For highway RPM, most drivers check 60, 65 and 70 mph.
- Enter your tire diameter in inches. Measure the mounted tire or work it out from the sidewall size. A 275/60R15 tire is about 28 inches tall. Our tire size calculator does the math for you.
- Enter your rear gear ratio. This is your axle ratio, such as 3.08, 3.42, 3.73 or 4.10.
- Click Calculate to see engine RPM at that speed.
How to Calculate Engine RPM From Gear Ratio and Tire Size
Engine RPM (revolutions per minute) is how many times the crankshaft turns in one minute. At a given speed, it depends on three things: tire diameter, rear gear ratio and the transmission gear you are in. The formula is:
RPM = (MPH × rear gear ratio × transmission gear ratio × 336) ÷ tire diameter
The constant 336 converts miles per hour and tire diameter in inches into revolutions per minute.
Worked Example
A street car with a 700R4 in overdrive (0.70), 3.73 rear gears and 28-inch tires at 65 mph:
65 × 3.73 × 0.70 × 336 ÷ 28 = about 2,037 RPM.
The same car in 3rd gear (1.00) at 65 mph turns about 2,909 RPM. That drop of nearly 900 RPM is why an overdrive transmission makes a big difference on the highway.
How to Convert RPM to MPH
To find your speed at a given RPM, flip the formula:
MPH = (RPM × tire diameter) ÷ (rear gear ratio × transmission gear ratio × 336)
Using the example above, 2,037 RPM × 28 ÷ (3.73 × 0.70 × 336) = about 65 mph.
Stock Transmission Gear Ratios
Use this chart if you want to run the numbers by hand or compare transmissions before you build.
| Transmission | 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th |
|---|---|---|---|---|---|---|---|---|
| GM TH350 | 2.52 | 1.52 | 1.00 | – | – | – | – | – |
| GM TH400 | 2.48 | 1.48 | 1.00 | – | – | – | – | – |
| GM 700R4 / 4L60E / 4L65E / 4L70E | 3.06 | 1.63 | 1.00 | 0.70 | – | – | – | – |
| GM 200-4R | 2.74 | 1.57 | 1.00 | 0.67 | – | – | – | – |
| GM 4L80E / 4L85E | 2.48 | 1.48 | 1.00 | 0.75 | – | – | – | – |
| GM 6L80E / 6L90E | 4.03 | 2.36 | 1.53 | 1.15 | 0.85 | 0.67 | – | – |
| GM 8L90E | 4.56 | 2.97 | 2.08 | 1.69 | 1.27 | 1.00 | 0.85 | 0.65 |
| Ford C4 / C6 | 2.46 | 1.46 | 1.00 | – | – | – | – | – |
| Ford AOD | 2.40 | 1.47 | 1.00 | 0.67 | – | – | – | – |
| Ford 4R70W | 2.84 | 1.55 | 1.00 | 0.70 | – | – | – | – |
| Ford E4OD / 4R100 | 2.71 | 1.54 | 1.00 | 0.71 | – | – | – | – |
| Ford 6R80 | 4.17 | 2.34 | 1.52 | 1.14 | 0.87 | 0.69 | – | – |
| Mopar 727 TorqueFlite | 2.45 | 1.45 | 1.00 | – | – | – | – | – |
Choosing a Rear Gear Ratio for Highway RPM
Changing tire size or swapping ring and pinion gears moves your cruising RPM. Taller tires lower RPM. A numerically higher rear gear, like going from 3.42 to 4.10, raises it. Run both your current and planned setups through the calculator before you buy parts.
- Street and highway builds usually run best with cruise RPM near the engine's torque peak, so the engine is not lugging or screaming at 70 mph.
- Drag and strip builds often use steeper rear gears for launch, then rely on an overdrive transmission to bring highway RPM back down.
- Bigger tires on a truck or 4x4 often call for steeper gears to restore the RPM you had with stock tires.
Want to go deeper? Read what rear end gear ratio is and how it is calculated, or see how we approach building a 4L60E with performance-friendly gear ratios.
RPM Calculator FAQ
How do you calculate engine RPM from speed?
Multiply your speed in MPH by your rear gear ratio, your transmission gear ratio and 336, then divide by your tire diameter in inches. The result is engine RPM.
How do I find my tire diameter?
Measure from the ground to the top of the mounted tire, or use the sidewall size. For a P-metric tire, tire diameter = (section width × aspect ratio × 2 ÷ 25.4) + wheel diameter. A 275/60R15 works out to about 28 inches.
Does tire size affect RPM?
Yes. A taller tire covers more ground per turn, so engine RPM drops at the same speed. A shorter tire raises RPM. Changing tire size also throws off your speedometer, which you can correct with our speedometer gear calculator.
Why is my actual RPM higher than the calculator shows?
The calculator assumes no slip. If your torque converter is not locked up, the tachometer will read a little higher than the calculated number. A lockup converter in overdrive brings real RPM close to the calculated figure.
What is a good highway RPM?
It depends on your engine's power band. Many street V8 builds cruise comfortably between about 1,800 and 2,500 RPM at highway speed. A high-stall converter or an aggressive cam may need a bit more.
Which transmission lowers highway RPM the most?
An overdrive transmission. Swapping a 3-speed like a TH350 or TH400 for a 700R4, 4L60E or 4L80E drops cruising RPM by about 25 to 30 percent with the same rear gears and tires. Browse Gearstar performance transmissions or get an estimate for your build.