Free Power to Weight Ratio calculator with clear step-by-step results.
Power to weight ratio is what actually predicts acceleration, because a given power has to accelerate a given mass. Two things are commonly fudged: the mass should include occupants and cargo rather than the bare kerb figure, and quoted power is measured at the crank, so 10-15% is lost through the gearbox and driveline on a conventional car. Both corrections push the honest ratio below the brochure number.
Power at the wheels
Wheel power = engine power × (1 − drivetrain loss)
Ratio
kW/t = wheel power ÷ (total mass ÷ 1000); W/kg = wheel power × 1000 ÷ total mass
Quoted power and mass figures vary by measurement standard and specification. Do not use this to plan driving on public roads beyond legal limits.
kW per tonne is the metric convention and hp per tonne the imperial one; W/kg is the same quantity again and is standard in cycling and aviation. All three are shown so the figure can be compared with whatever source you are reading.
Roughly 10-12% for a manual rear-wheel drive, 15-18% for an automatic all-wheel drive, and 5-8% for an electric vehicle with a single-speed reduction gear. Set it to zero if your power figure is already measured at the wheels.
It correlates strongly with acceleration but is not the whole story. Traction, gearing, aerodynamic drag and where in the rev range the power arrives all decide the actual time to speed.