RPM & Gear Speed Calculator
Speed in each gear and engine RPM at any speed from gear ratios, final drive and tyre size
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Calculates the road speed in every gear and the engine speed at any road speed from the gear ratios, the final drive and the tyre size. It also shows how a different final drive, sprocket or tyre changes those numbers, and has a bicycle gear table.
How to use
- On "Car and motorcycle", enter your "Gear ratios, first gear first" and the "Final drive ratio", or choose the motorcycle drive and enter the primary reduction and sprockets.
- Enter the "Tyre size" (or the measured circumference) and the engine speed range.
- Read the engine speed in top gear at the speed you set, the "Each gear" table and the sawtooth chart.
- Under "What if you change the final drive, sprockets or tyre", enter the new values to see the change in percent.
- For a bicycle, open "Bicycle" and enter chainrings, cogs, wheel and cadence.
How it works
- Speed (km/h) = rpm ÷ (gear ratio × final drive) × tyre circumference (mm) × 60 ÷ 1,000,000.
- After an upshift the road speed stays the same, so the engine speed falls to shift point × next ratio ÷ current ratio.
- Values are theoretical: wheel slip, air resistance and whether the engine has enough power are not considered. The example ratios are not those of a particular vehicle.
Questions and answers
- How do I calculate rpm at 100 km/h?
- Divide the speed by the tyre circumference to get wheel revolutions per minute, then multiply by the gear ratio and the final drive. With a 1,985 mm tyre and an overall ratio of 2.665 that is about 2,240 rpm.
- What does a shorter final drive do?
- A higher final drive number gives more engine speed at the same road speed and less top speed per gear. Going from 4.1 to 3.9 lowers the engine speed at the same road speed by about 5%.
- How much does one tooth on the front sprocket change?
- From 15 to 16 teeth the motorcycle goes about 6.7% faster at the same engine speed, and the engine turns about 6.3% slower at the same road speed.
- What are gear inches?
- The wheel diameter in inches multiplied by chainring teeth ÷ cog teeth. A 27-inch wheel with 48/16 gives 81 gear inches.