About the RPM to MPH Calculator
This RPM to MPH calculator converts engine speed into road speed using your transmission gear ratio, axle (final drive) ratio and tire diameter. It also works in reverse: enter a cruising speed and it shows the engine RPM you will be turning in that gear. A chart plots speed against RPM up to the redline so you can see the top speed in each gear.
It is useful when you are changing axle gears or tire sizes, choosing a final drive for towing or drag racing, checking whether an overdrive will bring highway RPM down, or correcting a speedometer after fitting bigger tires. Gear ratios are listed in your owner’s manual or transmission specifications, and the overall tire diameter can be calculated from the size on the sidewall.
The calculation assumes no clutch or torque-converter slip (a locked converter in automatics) and uses the nominal tire diameter. Real tires squat slightly under load, so actual speed is typically 1–3% lower than the result.
With the default inputs, the road speed at this rpm is 56.5 mph. Change any value above to recalculate instantly.
How to use the rpm to mph calculator
- 1Enter the engine RPM you want to convert.
- 2Enter the transmission ratio for the gear you are in and the axle ratio.
- 3Enter the overall tire diameter in inches.
- 4Enter a cruising speed to see the RPM it needs in that gear.
- 5Read the speed, cruising RPM and the speed-vs-RPM chart up to redline.
Formula and method
The engine turns the transmission, which reduces speed by the gear ratio G, and the differential reduces it again by the axle ratio A, so the wheels turn RPM ÷ (G × A) times per minute. Each wheel revolution moves the car one tire circumference, π × D inches. Multiplying by 60 minutes per hour and dividing by 63,360 inches per mile gives miles per hour.
Rearranged, the same relationship gives the engine RPM needed for a chosen speed. The constant 336.13 combines 63,360 ÷ (60 × π). The result assumes no slip and the tire’s nominal unloaded diameter.
- RPM
- Engine speed in revolutions per minute
- D
- Overall tire diameter in inches
- G
- Transmission gear ratio for the selected gear
- A
- Final drive (axle/differential) ratio
Worked examples
Overdrive 0.79, 3.73 axle, 28-inch tires at 2,000 rpm
The overall ratio is 0.79 × 3.73 = 2.947. At 2,000 rpm the wheels turn 678.7 rpm, and a 28-inch tire covers 87.96 inches per turn, giving 56.5 mph. Holding 70 mph needs about 2,476 rpm.
Truck with 31.6-inch tires, 4.10 gears, direct drive
With a 1:1 gear the overall ratio is just the 4.10 axle. At 2,500 rpm the 31.6-inch tires carry the truck at 57.3 mph, and 65 mph needs about 2,835 rpm.
Sixth gear 0.63 with 3.42 axle on 26.5-inch tires
A tall 0.63 sixth gear gives an overall ratio of 2.155. Every 1,000 rpm is worth 36.6 mph, so 1,800 rpm is 65.9 mph and 75 mph needs only about 2,050 rpm.
Frequently asked questions
How do you calculate MPH from RPM?+
Multiply RPM by tire diameter in inches, then divide by gear ratio × axle ratio × 336. For example, 2,000 rpm with a 28-inch tire, 1.00 gear and 3.73 axle gives 2,000 × 28 ÷ (3.73 × 336) ≈ 44.7 mph.
How do bigger tires change RPM at highway speed?+
A larger diameter travels farther per revolution, so the engine turns slower at the same speed. Going from 28-inch to 31-inch tires lowers highway RPM by about 10% — and makes the speedometer read about 10% low unless it is recalibrated.
What does a higher axle ratio do?+
A numerically higher ratio (e.g. 4.10 instead of 3.55) multiplies torque at the wheels for better acceleration and towing, but raises engine RPM at cruising speed, which usually costs some fuel economy and lowers top speed in each gear.
Where do I find my gear ratios?+
Transmission ratios are published in the owner’s manual, the manufacturer’s specifications or transmission model data sheets. The axle ratio is often on a tag on the differential cover or coded on the door-jamb sticker or window sticker.
Why is my real speed slightly different?+
Tires compress under load and grow slightly at speed, worn tread reduces diameter, and automatic transmissions can slip unless the torque converter is locked. Expect real speed to differ from the calculation by a few percent.