Transmission Calculator

Calculate a vehicle's road speed, wheel RPM, and total gear reduction from engine RPM, transmission gear ratio, axle (final-drive) ratio, and tire diameter.

Quick Facts

Speed formula
v = (Engine RPM × π × Tire Diameter) ÷ Total Gear Ratio
Total gear ratio combines the transmission ratio and axle ratio.
Total gear ratio
Transmission Ratio × Axle Ratio
A 1:1 transmission with a 3.73 axle gives a 3.73:1 total ratio.
Rule of thumb
MPH ≈ (RPM × Tire Diameter in) ÷ (Total Ratio × 336)
A common shop-floor shortcut that folds unit conversion into the constant 336.
Gearing tradeoff
Lower numerical ratio = taller gearing
Taller gearing raises top speed per RPM; shorter (higher-number) gearing raises torque and acceleration.

Your Results

Calculated
Vehicle Speed
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Road speed in mph
Vehicle Speed (metric)
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Road speed in km/h
Total Gear Ratio
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Transmission ratio × axle ratio
Wheel Speed
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Wheel rotation in RPM

Ready

Enter engine RPM, gear ratios, and tire diameter, then press Calculate.

How the Transmission Calculator Works

A vehicle's road speed depends on three things working together: how fast the engine spins, how much the transmission and axle reduce (or gear down) that spin before it reaches the wheels, and how big the tires are. This calculator applies the standard drivetrain speed formula — the same relationship used by racers and off-roaders to build gear-ratio charts — to convert engine RPM, gear ratios, and tire diameter directly into road speed and wheel rotational speed.

Deriving the speed formula

Power flows from the engine through the transmission, then through the axle (final-drive/differential) gears, and finally to the wheels. Each stage reduces rotational speed by its ratio, so the wheels turn at: Wheel RPM = Engine RPM ÷ (Transmission Ratio × Axle Ratio). The combined denominator is the Total Gear Ratio. Each wheel revolution covers one tire circumference (C = π × diameter), so road speed is v = Wheel RPM × π × Tire Diameter, converted to mph or km/h. A common automotive shortcut folds the unit conversion into a single constant: MPH ≈ (RPM × Tire Diameter in inches) ÷ (Total Ratio × 336).

Reading and choosing gear ratios

Gear ratios are written as input:output, such as 3.73:1. A lower numerical ratio (like 1.00 or 0.70 overdrive) is "tall" gearing — the output shaft turns nearly as fast as the input, giving higher top speed per RPM but less torque multiplication. A higher numerical ratio (like 3.5 in first gear, or a 4.10 axle) is "short" gearing — it multiplies torque for acceleration or towing at the cost of top speed. Swapping to a taller or shorter axle ratio, or changing tire diameter, shifts every gear's speed-per-RPM without changing the transmission itself.

Frequently Asked Questions

What is a transmission gear ratio?
A gear ratio is how many times the input shaft (from the engine) spins for every one rotation of the output shaft. A ratio of 3.5:1 means the engine turns 3.5 times for each single turn of the transmission's output shaft, multiplying torque at the cost of rotational speed.
How is total gear ratio calculated?
Total gear ratio equals the transmission (gearbox) ratio multiplied by the axle, or final-drive, ratio: Total Ratio = Transmission Ratio × Axle Ratio. This combined number tells you how many engine revolutions correspond to one revolution of the wheels.
Why does tire diameter affect the speed calculation?
A larger tire has a larger circumference, so it covers more ground per revolution than a smaller tire turning at the same RPM. Changing tire size without re-gearing changes both your true road speed and your speedometer/odometer accuracy.
What RPM should I use to estimate top speed?
Use the engine's redline (maximum safe RPM) in the highest available gear to estimate theoretical top speed, or use the RPM at your typical shift point to compare speeds gear-to-gear. Real-world top speed is usually lower due to aerodynamic drag and available power.

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