Gears in numbers
A bicycle’s gearing decides how far you go for each turn of the pedals, and therefore how hard each turn feels and how fast you can ride at a comfortable pedaling rate. Cyclists describe it in several ways: the gear ratio, gear inches, development and the gain ratio. They all start from the same three numbers: the teeth on the chainring, the teeth on the rear cog and the size of the wheel.
The formulas
- Gear ratio = chainring teeth ÷ cog teeth.
- Development = gear ratio × wheel circumference: the distance traveled per pedal turn.
- Gear inches = gear ratio × wheel diameter in inches.
- Speed = development × cadence (pedal turns per minute).
- Gain ratio = gear ratio × wheel radius ÷ crank length.
Worked example
A road bike in a 50 × 17 gear on 700×25c tires (circumference about 2,105 mm), pedaling at 90 rpm with 172.5 mm cranks:
- Gear ratio: 50 ÷ 17 = 2.94.
- Development: 2.94 × 2.105 m = 6.19 m (20.3 ft) per pedal turn.
- Gear inches: 77.6.
- Speed at 90 rpm: 6.19 × 90 = 557 m per minute, about 20.8 mph (33.4 km/h).
- Gain ratio: 5.71.
Typical gears
| Gear and wheel | Gear inches | Speed at 90 rpm |
|---|---|---|
| 32 × 51, 29-inch mountain | 18.0 | 4.8 mph |
| 34 × 34, 700×28c | 26.8 | 7.2 mph |
| 34 × 28, 700×25c | 32.0 | 8.6 mph |
| 39 × 21, 700×25c | 49.0 | 13.1 mph |
| 50 × 17, 700×25c | 77.6 | 20.8 mph |
| 53 × 11, 700×25c | 127.1 | 34.0 mph |
As a rough guide, gears under about 30 gear inches are for steep climbs, 60 to 80 for riding on the flat and over 110 for descents and sprints.
Wheel circumference
The calculator lists common sizes measured with the tire inflated and a rider aboard. The real value depends on the tire model, its width and its pressure, so for an accurate speed reading, measure yours: mark the tire and the floor, roll forward one full turn while sitting on the bike, and measure between the marks. The same number goes into a bike computer.
Cadence and gear choice
Most recreational cyclists pedal at 60 to 80 rpm and trained riders at 80 to 100. Choosing gears that keep your cadence steady, rather than grinding a big gear slowly, reduces strain on the knees. Use the calculator to check whether your lowest gear lets you climb your local hills above about 60 rpm, and whether your gear steps are close enough on the flat.
Gain ratio
Proposed by the bicycle mechanic and writer Sheldon Brown, the gain ratio compares how far the bike moves with how far your foot moves around the crank circle. It is the only measure that changes with crank length: longer cranks give more leverage and a slightly lower gain ratio for the same gear.