On a steep climb, riders often wish for an easier gear. But how much difference is there between a 28T, 32T, 36T, and 42T largest rear sprocket? This article compares the theoretical differences using gear ratio, cadence, wheel travel, and torque.
About this article: I am not a professional cyclist, bicycle mechanic, or competitive rider. I am a beginner learning about bicycles through riding, research, and calculation. The figures below are based on basic mechanics and formulas. Actual climbing performance varies with rider power, total mass, gradient, road surface, tires, wind, and riding position.
The lowest gear is determined by front teeth ÷ rear teeth
The basic gear-ratio formula is:
Gear ratio = front chainring teeth ÷ rear sprocket teethFor this comparison, assume a 40T front chainring and a tire circumference of 2.155 m, roughly representative of a 700×32C setup.
| Combination | Gear ratio | Relative to 40×28 | Character |
|---|---|---|---|
| 40T×28T | 1.43 | 100% | Relatively harder |
| 40T×32T | 1.25 | 87.5% | About 12.5% easier |
| 40T×36T | 1.11 | 77.8% | About 22.2% easier |
| 40T×42T | 0.95 | 66.7% | About 33.3% easier |
A lower gear ratio produces less road speed at the same cadence, but it reduces the crank torque required to overcome a given resistance at the rear wheel.
How fast at 60 and 80 rpm?
The theoretical road speed can be calculated as:
Speed (km/h) = tire circumference (m) × gear ratio × cadence (rpm) × 60 ÷ 1000| Gear | 60 rpm | 80 rpm |
|---|---|---|
| 40T×28T | about 11.1 km/h | about 14.8 km/h |
| 40T×32T | about 9.7 km/h | about 12.9 km/h |
| 40T×36T | about 8.6 km/h | about 11.5 km/h |
| 40T×42T | about 7.4 km/h | about 9.9 km/h |
Example: With 40T×42T at 60 rpm, 2.155 × (40÷42) × 60 × 60 ÷ 1000 ≈ 7.39 km/h.
Easier gears multiply rear-wheel torque
Ignoring drivetrain losses, rear-wheel torque is proportional to rear sprocket teeth divided by front chainring teeth. With the same crank torque, the torque multiplication factor is 0.70 for 40×28 and 1.05 for 40×42.
| Gear | Rear/front | Relative to 28T |
|---|---|---|
| 40×28 | 0.70 | 1.00× |
| 40×32 | 0.80 | about 1.14× |
| 40×36 | 0.90 | about 1.29× |
| 40×42 | 1.05 | about 1.50× |
So, for the same crank torque, 40×42 produces about 50% more rear-wheel torque than 40×28. The trade-off is that road speed at the same cadence is about 33% lower.
An easier gear does not create more power
This distinction matters. Changing gear does not increase the rider’s available power. A simplified expression for the power needed to overcome gravity on a climb is:
P ≈ m × g × v × gradientFor example, with an 80 kg total system mass, an 8% gradient, and a speed of 10 km/h, gravity alone requires about 174 W. Rolling resistance, aerodynamic drag, and drivetrain losses add to that.
The main purpose of an easier gear is to reduce the force required per pedal revolution and make it easier to maintain a usable cadence. An easier gear is not the same thing as producing more power.
What happens when the lowest gear ratio is below 1.0?
A 1.0 gear ratio means the rear wheel turns about once for each crank revolution. With 40T×42T, the ratio is about 0.95, so the rear wheel turns less than once per crank revolution. That sacrifices speed but can make steep gradients much easier to pedal.
If most riding is on flat roads, such a low gear may be used rarely. Cassette spacing, derailleur compatibility, and the rest of the gear range also matter.
Conclusion: 28T to 42T is a substantial change
With a 40T chainring, changing the largest rear sprocket from 28T to 42T lowers the gear ratio from 1.43 to 0.95. At the same cadence, theoretical speed drops by about 33%, while rear-wheel torque for the same crank torque rises by about 50%.
On climbs, the key issue is not maximum speed but whether the rider can maintain a sustainable cadence. If cadence collapses because the lowest gear is too hard, a larger low-gear sprocket can make a meaningful mechanical difference.
