Power-to-weight ratio explained
Why power-to-weight predicts performance better than horsepower alone, how to calculate it in hp/ton and kg/hp, and how to compare cars fairly.
Horsepower gets the headlines, but power-to-weight ratio is what actually predicts how a car accelerates. A 300 hp car that weighs a tonne will out-run a 400 hp car that weighs two, because every horsepower has less mass to move.
How it's expressed
Two conventions dominate, and they run in opposite directions:
- Power per weight — e.g. hp per ton or kW per tonne. Higher is faster.
- Weight per power — e.g. kg per hp or lb per hp. Lower is faster.
They describe the same thing inverted. A car at 250 hp/ton is the same as 4 kg/hp. Pick one convention and stick to it when comparing.
The calculation
hp per ton = power (hp) ÷ weight (tonnes)
So 320 hp in a 1,250 kg car is 320 ÷ 1.25 = 256 hp/ton. Flip it for kg/hp: 1,250 ÷ 320 = 3.9 kg/hp.
Why weight punches above its weight
Adding power and removing weight both improve the ratio, but weight loss also helps braking, cornering, tyre wear and fuel use — power only helps in a straight line. This is the whole philosophy behind lightweight sports cars: 200 hp in an 1,100 kg chassis feels and performs better everywhere than the number suggests.
Comparing fairly
- Use the same weight basis. Kerb weight, dry weight and "with driver" all differ. Manufacturers often quote dry; real-world figures should add fluids and a driver (~75 kg).
- Use the same power basis. Crank (flywheel) hp is higher than wheel hp by roughly 15% on the drivetrain loss. Compare crank-to-crank or wheel-to-wheel, not one against the other.
- Remember it's a straight-line proxy. Power-to-weight predicts acceleration well but says nothing about grip, aero or gearing.
Rough benchmarks (crank hp, with driver)
- Warm hatch: ~120–160 hp/ton
- Hot hatch / sporty saloon: ~180–230 hp/ton
- Serious sports car: ~250–330 hp/ton
- Supercar territory: ~400 hp/ton and up
FAQ
Is power-to-weight better than horsepower for comparing cars?
Yes for acceleration. Two cars with the same power-to-weight ratio accelerate similarly regardless of their absolute power, which is why it is the fairer single number for comparing performance.
Should I use crank or wheel horsepower?
Either, as long as you are consistent. Crank (flywheel) figures are higher than wheel figures by the drivetrain loss (~15% typical). Comparing a crank figure against a wheel figure flatters the crank car unfairly.
Does losing weight or adding power help more?
For the ratio itself they are interchangeable, but weight loss also improves braking, cornering and tyre life, while power only helps straight-line speed. Gram for gram, weight reduction usually improves the whole car.