Bike drivetrain basics
What does a bike gear ratio mean?
A bicycle's gear ratio compares the number of teeth on the front chainring with the number of teeth on the selected rear cog. In a conventional chain drivetrain, divide front chainring teeth by rear sprocket teeth. A 50-tooth chainring and a 17-tooth rear cog give 50 ÷ 17 ≈ 2.94.
At that ratio, each full crank turn makes the driven wheel complete approximately 2.94 revolutions. A larger front chainring or smaller rear cog moves the bike farther per crank revolution, but is generally harder to pedal at the same resistance.
How gearing, cadence, and wheel size fit together
The bike's forward distance per crank turn equals its wheel rollout circumference multiplied by the gear ratio. Multiplying that distance by cadence produces meters per minute. Multiply by 60 and divide by 1,000 to get theoretical km/h.
| Setup | Ratio | Distance / crank turn* | Speed at 90 rpm* |
|---|---|---|---|
| 34 / 28 | 1.21 | 2.55 m | 13.77 km/h |
| 50 / 17 | 2.94 | 6.18 m | 33.35 km/h |
| 50 / 14 | 3.57 | 7.50 m | 40.50 km/h |
*Examples assume a 2.10 m measured wheel circumference; actual wheel rollouts differ.
Cadence is not the same as speed
Cadence counts pedal revolutions per minute, while speed is the bicycle's motion over the ground. At equal cadence, a higher gear ratio gives a higher theoretical speed. It does not mean a rider can produce enough force to maintain that cadence on a hill or into a headwind.
How to measure wheel rollout
Mark a point on the tire, roll the loaded bicycle one full wheel revolution in a straight line, and measure the distance between marks. Convert centimeters to meters. Check tire pressure and load because the effective rolling circumference can change.