How gearing and road speed relate
The gear ratio is driven teeth divided by driving teeth. The overall ratio multiplies that by the final drive ratio, so a 3.0 gear with a 3.7 final drive gives 11.1 overall, meaning the engine turns 11.1 times for each turn of the wheel.
Wheel speed is engine speed divided by the overall ratio. Road speed is the wheel speed multiplied by the wheel circumference, converted from inches per minute into miles per hour.
Worked example
A 12 tooth driving sprocket and a 36 tooth driven sprocket give a ratio of 3.00. With a 3.7 final drive the overall ratio is 11.10, so at 3,000 rpm the wheel turns 270 times a minute. A 26 inch wheel has a circumference of 81.68 inches, which gives 20.9 mph or 33.6 km/h.
These figures are illustrative. Enter your own tooth counts, final drive and wheel diameter.
Why the wheel diameter matters
The gear ratio alone does not fix road speed. A larger wheel covers more ground per revolution, so the same gearing gives a higher speed and a lower effective ratio.
Use the overall wheel and tire diameter, not the rim size. A 17 inch rim with a 225/45 tire is about 25 inches overall, which is the figure this calculator needs.
Where the numbers come from
Tooth counts are stamped on sprockets or printed in the handbook for a gearbox gear. The final drive ratio is the differential ratio, often on a plate or in the specification, and is 1 for a direct-drive hub.
For a bicycle, the driving teeth are the front chainring and the driven teeth are the rear sprocket, with a final drive of 1. The result then shows the speed that cadence would produce for a given wheel.
Limits of this calculator
It assumes no slip, a locked converter and a tire that keeps its nominal diameter. Real speed varies with load, gradient, wear and tire growth at speed.
It calculates one gear at a time. For the speedometer error after a wheel change, use the speedometer calculator.
For the dial error after a wheel change, use the US speedometer calculator.
For UK spelling and tyre terminology, use the UK gear ratio calculator.
Inputs for the US gear ratio calculator
Engine speed
The engine or motor speed you want to convert into road speed, in revolutions per minute.
Driving sprocket or gear teeth
Teeth on the part that drives, such as the transmission output gear or the front chainring.
Driven sprocket or gear teeth
Teeth on the part being driven, such as the rear sprocket or the transmission input gear.
Final drive ratio
Differential or hub reduction ratio. Enter 1 when there is no separate final drive.
Wheel diameter
Overall wheel and tire diameter in inches, not the rim size on its own.
CALCULATOR QUESTIONS
Gear ratio calculator US questions
Is a higher gear ratio better for acceleration?
A numerically higher ratio multiplies torque more, which usually helps acceleration and lowers top speed for the same engine speed.
Why does my calculated speed differ from the speedometer?
Speedometers read high by design, and tire wear, load and converter slip all change the real figure.
Can I use this for a bicycle?
Yes. Use the chainring and rear sprocket teeth with a final drive of 1, and the overall wheel diameter including the tire.
What is the difference between gear ratio and final drive?
The gear ratio is the transmission or sprocket pair. The final drive is the differential, and the overall ratio multiplies the two.