To check how hard under-inflated tyres will hit your wallet, we used satellite-navigation-based equipment to measure precisely how far a car travels when freewheeling to a halt from a set speed.
We used four cars of varying sizes and weights - a Kia Picanto, Volkswagen Golf GTI, Ford Mondeo and Land Rover Freelander - and measured the coasting distance from 30mph with correctly inflated tyres. We then repeated the experiment with the tyres at half the recommended pressure.
Our tests show coasting distances were reduced by between 14% and a whopping 27%. This extra drag means you could be shelling out an extra 390 every year in fuel costs, and be pumping nearly a tonne of extra carbon dioxide into the atmosphere - all for the sake of a minute's maintenance each week.
Basically, the flatter your tyres the more you'll spend on fuel. The cost will hit you and the environment even harder if you've got a thirsty car or do lots of miles.