Pick a mode, then enter the teeth / ratios and tire size. The gear ratio, distance travelled per turn and resulting speed are solved in your browser — nothing is sent anywhere. Bicycle uses front÷rear teeth; car/motorcycle multiplies primary × gear × final reduction; RC uses (spur÷pinion) × internal ratio.
A gear ratio calculator that works from teeth counts, reduction ratios and tire size. Pick a mode — Bicycle, Car/Motorcycle or RC — and the inputs and formula switch to match. Bicycle mode divides front (chainring) teeth by rear (cog) teeth, then uses the rolling circumference (presets like 700×25C plus a custom value) and cadence (rpm) to show gear inches, the distance travelled per crank revolution (development) and your speed in km/h. Car/Motorcycle mode multiplies primary × gear × final drive into a total reduction, then uses tire diameter and engine rpm to give wheel rpm and speed. RC mode computes the final drive ratio (FDR) as (spur ÷ pinion) × internal ratio, then uses tire diameter and motor rpm to show rollout (distance per motor turn) and speed. Handy for choosing a fixed-gear/single-speed ratio, simulating speed after a final-drive change, or dialing in RC FDR and rollout. Everything is computed in your browser; the values you enter are never sent anywhere.
How to use
- Choose a mode: Bicycle, Car/Motorcycle or RC.
- Enter teeth or reduction ratios (front/rear for a bicycle), plus tire size and rpm.
- Gear ratio, distance per turn (rollout) and speed are solved automatically — nothing is sent.
FAQ
Are the teeth and tire values I enter sent anywhere?
No. The gear ratio, distance-per-turn and speed calculations all run entirely in your browser. The values you type are never sent to any server or API — it all stays on your device.
How is the bicycle gear ratio calculated?
It divides the front (chainring) teeth by the rear (cog) teeth. Multiplying by the rolling circumference gives the development (distance per crank revolution), and multiplying by cadence (rpm) gives your speed in km/h. Gear inches is gear ratio × wheel diameter in inches.
Can I work out speed after a final-drive change on a car or motorcycle?
Yes. It multiplies primary × gear × final into a total reduction, divides engine rpm by it for wheel rpm, and uses the tire diameter for speed. Change only the final drive ratio to compare speed at the same rpm before and after the swap.
What are FDR and rollout in RC mode?
FDR (final drive ratio) is spur teeth ÷ pinion teeth × internal ratio — how far the tire turns per motor revolution. Rollout is tire circumference ÷ FDR, the distance the car travels per motor turn (mm). Use them to dial in gearing when changing pinion or spur.