It might not only be your money you throw away by not pumping up your tyres - it could be your life or someone else's.
Driving with under-inflated tyres causes extra flex in the rubber, and increased resistance with the road. This added friction generates a build-up of heat, which massively increases your risk of a dangerous tyre 'blow-out', which is often closely followed by the driver losing control of the car.
Indeed, tyre manufacturers estimate that three-quarters of all blow-outs are triggered by under-inflated tyres, and account for 6% of all fatal motorway accidents.
Failing to keep your tyres pumped up also devastates a car's handling. You'll have much less chance of avoiding an accident should you need to swerve suddenly, and your car may even be unstable during normal cornering.
We used a basic cone slalom test to rate under-inflated tyres.
First, we found a speed at which the cars could safely negotiate the cones with correct tyre pressures. Then we dropped the pressures by half and attempted the slalom again.
The result was alarming. The cars' inability to change direction quickly meant they often ran wide or hit a cone.
What Car?'s road test editor, Pete Tullin, performed the handling tests and said: 'I expected under-inflated tyres to affect handling, but was shocked by how much they did. The cars couldn't change direction properly, the handling became unpredictable and I couldn't maintain anywhere near the same speed without sliding wide or annihilating a cone. All that was with the stability control on.'
We also compared the braking performance of correctly inflated tyres with those at half pressure. Although the actual stopping distances from 30mph and 70mph didn't vary significantly, our testers noted that the vehicles were more difficult to control under heavy braking.
This wasn't a problem at the empty test track, but would be a different story if a child ran in front of you in a busy town centre.