From revs to road speed

A lower gear offers more turning effect at the output. What does it trade to get that?

A transmission changes the relationship between engine speed and wheel speed. A low gear delivers more wheel torque for a given engine torque. A higher gear lets the car travel faster at the same engine speed.

In an ideal reduction, output speed equals input speed divided by the ratio, while output torque is multiplied by that ratio. Real gears have losses. The final drive adds another ratio between the transmission and wheels.

Manual gearboxes use a clutch to connect the engine. Many conventional automatics use a fluid torque converter, often with a lock-up clutch to reduce slip. Dual-clutch transmissions use a different arrangement. The ratios below are teaching examples, not BMW transmission specifications.

1. 1
   3 : 1

   A teaching reduction: 3,000 rpm in → 1,000 rpm out; ideal torque multiplies by three.

2. 2
   2 : 1

   At 3,000 rpm input, output is 1,500 rpm; ideal torque multiplies by two.

3. 3
   1 : 1

   Direct ratio: input and output turn at the same speed before the final drive.

4. 4
   0.75 : 1

   Overdrive: 3,000 rpm in → 4,000 rpm out; ideal output torque is lower.

Think it through

A 3:1 reduction receives 3,000 RPM and 100 Nm. Predict both output values before opening the explanation. Ignore losses for this example.

Show the reasoning

3,000 ÷ 3 = 1,000 RPM; 100 × 3 = 300 Nm. More torque comes with less speed. The ideal gearset has not created power.