34×21 ratio 1.619 — 3.408 m per stroke
A 34-tooth chainring with a 21-tooth sprocket gives a ratio of 1.619. One pedal stroke carries you 3.408 m, and at 90 rpm that is 18.4 km/h.
Speed
Speed is the ratio multiplied by cadence. Grinding a big gear slowly and spinning a small one quickly give the same speed, but legs last far longer doing the second — which is why most riders pick the gear that keeps them near 80 to 90 revolutions a minute.
- Cadence 60 rpm12.3 km/h
- Cadence 80 rpm16.4 km/h
- Cadence 90 rpm18.4 km/h
- Cadence 100 rpm20.4 km/h
- Chainring
- 34T
- Sprocket
- 21T
- Gear ratio
- 1.619
- Per pedal stroke
- 3.408 m
- Gear inches
- 42.6
Different teeth, same gear
A 53×19 and a 39×14 share almost the same ratio, so one pedal stroke carries you the same distance despite wholly different tooth counts. That is why "which gear" means something different on every bike, and why the ratio is the only thing you can compare. Each cell lists the other combinations that feel the same.
Nearby cells
Same chainring, other sprockets
Same sprocket, other chainrings
Worth knowing
- The ratio is the chainring divided by the sprocket — bigger means harder and further.
- Two combinations with the same ratio are the same gear, whatever the tooth counts.
- Speed is ratio times cadence, so neither number settles it alone.
- These figures assume 700×25C wheels with a 2,105 mm circumference.
Common questions
Q. What is the ratio of 34×21?
1.619. One pedal stroke carries you 3.408 m, which is 42.6 gear inches.
Q. How fast is this gear?
At 90 rpm it works out to 18.4 km/h. Speed scales with cadence, so 60 rpm gives two thirds of that and 100 rpm a little more.
Q. Is there another combination that feels the same?
52×32 share almost the same ratio.
Q. What happens one sprocket lighter?
You land on 34×23: the pedals ease off, but each stroke carries you less far.