Driving your car Driver tools

Want to work out your car's true mpg, or whether you'll be better off with a diesel car? Our interactive tools make doing the sums easy.

Does your car match the maker's mpg claims?

Fuel economy calculator

How efficient is your car? Use our fuel calculator to check your mpg


Petrol vs diesel

Which is cheaper to run? We do the sums so you don't have to

Power conversion from PS to BHP to KW

Power converter: bhp, PS and kW

Convert quickly and easily between bhp, kW and PS.

Litres to gallons converter NEW

Litres to gallons converter

Quick litres-to-gallons conversion tool from Which?.

  • Speed converter (mph to kmh)

    Speed conversion from kmh to mph

    Free speed calculator tool

  • Car emissions - the worst offenders

    Car exhaust emissions

    In our realistic tests, 95% of diesel cars, and 10% of petrol cars, pump out more NOx than limits allow. The majority of petrol cars we’ve tested also exceed carbon monoxide (CO) regulations, and some hybrid cars are not as innocent as claimed.