It sounds like this“A low gear gives the engine extra power, which is why you pull away so hard in first.”
Thinking a low gear gives the engine extra power
Why it feels right
You can feel it. Pull away in first gear and the car surges forward, then in fifth it feels lazy. It seems obvious that the engine is working harder, or that the gearbox gives it a boost. Everyday words like 'a low gear for more power' and 'gearing up' for strength keep the idea alive, and a long lever or a bike's low gear does feel like getting something for nothing.
Why it’s wrong
Gears can't add power. A 12 to 1 ratio turns 100 N m into 1,200 N m at the wheels but slows them to a twelfth of the engine's speed, so the power is the same. First gear gives a big push at low road speed, not more power.
A worked example
A gearbox is a lever. A long lever lets a small push move a heavy thing, but you have to move your hand a long way. You never get more work out than you put in.
With numbers: an engine makes 100 N m at 6,000 rpm. The gearbox and final drive together have a ratio of 12 to 1. The wheels get 100 x 12 = 1,200 N m of torque, but they turn 12 times slower, at 500 rpm.
Compare the power. Engine: 100 N m x 628 rad/s = 62.8 kW. Wheels: 1,200 N m x 52.4 rad/s = 62.8 kW. Exactly the same (less a few per cent lost to friction in the gearbox).
First gear feels strong because force at the road is torque divided by wheel radius, and a big ratio makes that force large. The price is speed: first gear tops out at a low road speed. Gears trade speed for push. They never add power.
How to catch yourself
- You say a low gear 'has more power'.
- You think a gearbox boosts what the engine produces.
- You expect gears to make a weak engine as strong as a powerful one.
What to hold instead
A gearbox multiplies torque by its ratio and divides speed by the same ratio, so the power out equals the power in, apart from small losses. Low gears give a big push at low road speed in exchange for a low top speed; they don't add power.
Where you’ll meet it
- Understanding why a bicycle's low gear helps on hills but makes you spin fast.
- Seeing why a lorry has so many gears.
- Reading why a motorbike's gearing is chosen for the engine's rev range.
Questions people ask
- Does a lower gear give more power?
- No. A lower gear gives more torque at the wheels, and less speed, in exactly the same proportion, so the power is unchanged apart from small friction losses. It lets the engine run in its best rev range for the road speed.
- Why do cars need more than one gear?
- Because an engine makes useful power only over a range of rev speeds, and the wheels must turn over a much wider range. Gears let the engine stay in its good range while the car's speed changes from walking pace to motorway speed.