What Is Bicycle Gear Ratio? Calculating Speed Differences from Tooth Counts

Gear ratio is one of the basic numbers used to understand bicycle speed and climbing ability. A higher ratio moves the bike farther for each crank revolution at the same cadence, while a lower ratio is easier to turn. This article shows how to calculate bicycle gear ratio from tooth counts and then estimate theoretical speed using wheel circumference and cadence.

What is gear ratio?

Bicycle gear ratio is calculated by dividing the number of teeth on the front chainring by the number of teeth on the rear sprocket.

Gear ratio = front chainring teeth ÷ rear sprocket teeth

What does a higher or lower ratio mean?

A higher ratio produces more wheel revolutions per crank revolution and is therefore suited to higher speeds. However, it generally requires more pedal torque to accelerate or climb. A lower ratio reduces speed at the same cadence but makes the gear easier to turn, which is useful for starts and hills.

How to calculate theoretical bicycle speed

If wheel circumference is C in meters, gear ratio is G, and cadence is R in rpm, theoretical speed V in km/h can be estimated as follows.

Speed [km/h] = wheel circumference [m] × gear ratio × cadence [rpm] × 60 ÷ 1000

For this example, assume a wheel circumference of 2.155 m, roughly representative of a 700×32C setup, a 40T front chainring, and a cadence of 90 rpm.

40T chainring with rear sprockets from 11T to 36T

At the same cadence, speed is proportional to gear ratio

If wheel circumference and cadence stay constant, theoretical speed changes in direct proportion to gear ratio. For example, 40T×11T gives a ratio of 3.64, while 40T×22T gives 1.82. The first ratio is exactly twice the second, so the theoretical speed is also twice as high at the same cadence.

40 ÷ 11 = 3.64
40 ÷ 22 = 1.82
3.64 ÷ 1.82 = 2.00

A higher gear does not automatically make you faster

A higher ratio only means that the bicycle would travel faster if the rider could maintain the same cadence. In real riding, aerodynamic drag rises rapidly as speed increases and the required power also increases. If a gear is too high to spin efficiently, a lower gear may actually produce a higher real-world speed.

Lower ratios help on climbs

On hills, maintaining a sustainable cadence is often more important than maximizing speed. A large rear sprocket such as 36T lowers the gear ratio and reduces the pedal force needed for a given crank torque demand. The trade-off is lower speed at the same cadence.

Wheel circumference also changes speed

Two bikes with the same gear ratio and cadence can still have different theoretical speeds if their wheel circumferences differ. Tire width, tire model, pressure, and actual inflated dimensions can all slightly change circumference, so exact comparisons should use the measured rollout distance when possible.

Summary

Bicycle gear ratio is simply front teeth divided by rear teeth. A higher ratio gives more speed at the same cadence, while a lower ratio is easier to turn. By combining gear ratio with wheel circumference and cadence, you can estimate theoretical bicycle speed and compare different gearing setups numerically.