Tools · Gear ratio calculator
Gear Ratio Calculator
Work out your road speed from engine RPM, transmission gear, final drive, and tire size, or run it backward to see what RPM a cruise speed needs. The result updates live in both mph and km/h.
Reverse: at 65.4 mph in this gearing the engine turns 3000 RPM.
The calculator
Enter the four numbers that set your road speed: engine RPM, tire diameter in inches, the ratio of the transmission gear you are in, and the final drive (axle) ratio. The speed appears in mph and km/h at once, and the line below shows the reverse: the RPM the engine turns at that speed in that gear. Change any field and everything recomputes.
The defaults (3000 RPM, a 26 inch tire, a 1.00 direct gear, and a 3.55 final drive) land at 65.4 mph. Drop into an overdrive gear, fit a taller tire, or pick a lower axle ratio and the speed per RPM climbs.
How gearing sets your speed
Speed is a chain of multiplications and divisions. The engine spins at some RPM. The transmission gear divides that down by its ratio, then the final drive divides it down again by its ratio. What is left is how fast the wheels turn. Each wheel turn moves the car forward by its circumference, which is pi times the tire diameter. Multiply rotations per minute by circumference and scale to an hour and a mile, and you have road speed.
The formula used here is mph = (RPM x tire diameter x pi x 60) divided by (gear ratio x final drive x 63,360). The 60 turns per-minute into per-hour, and 63,360 is the number of inches in a mile. Two things move the answer the most. A taller tire raises speed per RPM because each rotation covers more ground, and it also throws off the factory speedometer, which still assumes the original diameter. Lower combined gearing (a smaller gear times final drive product) does the same on the math side without touching the dash.
Speed by gear at 3000 RPM
With a 3.55 final drive and a 26 inch tire, here is the road speed in each transmission gear at a steady 3000 RPM.
| Gear | Ratio | mph | km/h |
|---|---|---|---|
| 1st (low) | 3.00 | 21.8 | 35.1 |
| 2nd | 2.00 | 32.7 | 52.6 |
| 3rd | 1.50 | 43.6 | 70.1 |
| 4th (direct) | 1.00 | 65.4 | 105.2 |
| 5th (overdrive) | 0.80 | 81.7 | 131.5 |
Tire size and speedometer error
Tire diameter sits right in the middle of this calculation, so any change to it changes both the speed you compute here and the speed your dashboard reports. Fit a taller tire and each revolution covers more ground: you are actually going faster than the factory speedometer shows, because it was calibrated for the original diameter. Fit a shorter tire and the dash reads high. If you have changed wheel or tire sizes, run the numbers through the tire size calculator to see the new overall diameter and how far the speedometer is off, then bring that diameter back here to get your real speed per RPM.
FAQ
How do you calculate speed from RPM and gear ratio?
Multiply engine RPM by the tire circumference (pi times the tire diameter in inches) and by 60, then divide by the transmission gear ratio times the final drive ratio times 63,360. That gives mph. Multiply by 1.609344 for km/h. The engine spins the gearbox, the gearbox and axle reduce it through their combined ratio, and the wheels turn the rest into distance.
What is a final drive ratio?
The final drive, or axle ratio, is the reduction inside the differential between the driveshaft and the wheels. A 3.55 final drive means the driveshaft turns 3.55 times per wheel turn. It multiplies with the transmission gear to give the total reduction. A higher number gives stronger acceleration and higher cruising RPM; a lower number gives relaxed, efficient highway cruising.
Does tire size affect my speed and RPM?
Yes. A taller tire covers more ground per revolution, so at the same RPM and gearing you go faster, and at any given speed the engine turns slower. A shorter tire does the reverse. Because the speedometer is calibrated for the original diameter, taller tires make it read low and shorter tires make it read high.
What RPM should I cruise at on the highway?
Most modern cars sit around 1,800 to 2,500 RPM at 70 mph in top gear. Older or shorter-geared vehicles and trucks can sit at 3,000 RPM or more. Enter your top-gear ratio, final drive, and tire size above, and the reverse line shows the RPM your cruise speed needs.
How does gearing affect my speedometer?
The speedometer converts rotations to speed using a fixed assumption about tire diameter and axle ratio. Change either and the reading is off: a numerically higher axle ratio or shorter tires read high, while taller tires or a lower axle read low. A GPS speed reading is the way to check it.
Related tools
- Tire size calculator: compare tire diameters and the speedometer error from a wheel or tire swap.
- 0-60 mph calculator: turn an acceleration time into average g-force, or the reverse.
- Speed converter: switch between mph, km/h, knots, m/s, fps, and Mach.
- Live GPS speedometer: your real speed in the browser, no install.