Enter your driver gear teeth and driven gear teeth to calculate your gear ratio — then optionally add engine RPM, tire diameter, and axle ratio to get output RPM and vehicle speed (MPH). Perfect for ring-and-pinion selection, electric scooter builds, and transmission tuning.
Gear Ratio
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Output Shaft RPM
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Vehicle Speed
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Mechanical Advantage
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Torque Multiplication
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Gear ratio is calculated by dividing the number of teeth on the driven (output) gear by the number of teeth on the driver (input) gear. For example, if your driven gear has 41 teeth and your driver gear has 9 teeth, your gear ratio is 41 ÷ 9 = 4.56:1. A higher ratio means more torque but lower speed.
The ideal gear ratio depends on your application. Higher ratios (e.g. 4.56:1 or 4.88:1) provide more torque and acceleration, which suits trucks, off-road vehicles, and towing. Lower ratios (e.g. 3.08:1 or 3.55:1) offer better fuel economy and highway cruising. Your engine's power band, tire size, and transmission ratios all factor into the best selection.
Larger tires effectively lower your final drive ratio because each wheel revolution covers more ground. This means your engine has to work harder at low speeds. When you increase tire diameter, you may need to re-gear (install a numerically higher axle ratio) to restore acceleration and maintain proper engine RPM at highway speeds.
Gear ratio describes the speed relationship between driver and driven gears (how many times the input shaft turns for each output shaft revolution). Mechanical advantage specifically refers to the torque multiplication — a 4:1 gear ratio provides 4x the torque at the output shaft compared to the input, at 1/4 of the rotational speed.
Vehicle speed (MPH) = (Engine RPM × Tire Diameter × 0.002975) ÷ (Gear Ratio × Transmission Ratio). The constant 0.002975 accounts for the conversion from inches per minute to miles per hour (π ÷ 63360 × 60). This formula assumes no tire slip and uses the overall tire diameter, not the wheel diameter.
A 1:1 gear ratio means the input and output shafts rotate at exactly the same speed with no mechanical advantage — one revolution in equals one revolution out. This is commonly called 'direct drive' and is typical of many transmissions in their top gear. There is no torque multiplication, but there is also no speed reduction.
Yes. Enter your motor sprocket teeth as the Driver Gear Teeth and your wheel sprocket teeth as the Driven Gear Teeth. Then enter your motor's no-load RPM and wheel diameter to calculate top speed. Keep in mind that real-world top speed will be somewhat lower due to motor load, friction, and rider weight.
Changing your ring-and-pinion (axle) ratio is one of the most significant modifications you can make to a vehicle's drivetrain. A numerically higher ratio (e.g. switching from 3.55 to 4.10) improves low-speed torque and towing ability but increases engine RPM at highway speed. A numerically lower ratio reduces RPM at cruising speed, improving fuel economy but reducing pulling power.