Power-to-weight ratio

What it's for

The power-to-weight ratio (kg per hp) is a performance index: the lower it is, the livelier the vehicle. Handy to compare cars and motorbikes.

A2 license

A2 motorbikes must have at most 35 kW of power and a power-to-weight ratio no higher than 0.2 kW/kg. The tool tells you if the vehicle qualifies.

Why it matters more than power alone

Two hundred horsepower in an 1,800 kg saloon and two hundred in a 1,100 kg sports car are two different cars: in acceleration and through corners the second wins, and not by a little. Power-to-weight puts both numbers into one, which is why it is the index used to compare vehicles that have nothing else in common.

The rough rule works well: below 5 kg per horsepower you are talking about a sports car, around 10 a lively normal car, above 15 a vehicle built to carry rather than to accelerate. A lorry is beyond 60.

Which weight goes into the sum

The figure shifts a lot depending on the weight used: kerb weight, running order weight (fluids and a full tank) or fully laden weight. The same car can read 8 or 11 kg/hp depending on the choice, so two figures being compared must use the same definition.

In practice, running order weight plus the driver is the most honest compromise: it is the condition in which the vehicle is actually driven. Adding four passengers and luggage can worsen the ratio by 20%, and indeed a loaded car feels different immediately.

Where the number is a legal limit

For the A2 licence the ratio is not a sporting index but a limit: the motorcycle must stay within 35 kW and not exceed 0.2 kW/kg, and this second constraint rules out very light machines restricted to exactly 35 kW.

The double rule applies too: a motorcycle may be restricted to 35 kW only if it originally made no more than 70. It is not the final power that decides, it is the starting one, and it is the point people get wrong most often when buying second-hand.

Nearby tools

For the motorcycles allowed on the intermediate licence there is A2 licence motorcycles. On the car side, Power surtax calculator says what power costs in tax and Yearly car cost what everything else costs.