Under-inflated tyres could cost you £450 a year
Exclusive What Car? testing highlights the cost and danger of driving with too little air in your car's tyres...
It’s widely recommended you check your car’s tyre pressures once a month, but the reality is that many people don’t do this. In fact, one in five drivers never check their tyres, instead relying on car servicing departments and MOT stations to do this for them once a year. As a result, more than half of the cars being driven on UK roads have under-inflated tyres.
Exclusive research conducted by What Car? shows that there are implications for both safety and running costs of driving with incorrectly inflated tyres. Driving with too little air in your car’s tyres adversely affects fuel economy, and it is also a serious safety concern.
We conducted a laboratory test of three tyres to measure their rolling resistance, which is the friction a tyre generates as it rolls along the road. The higher the rolling resistance, the more energy is needed to keep the car at a given speed. This has a big effect on fuel economy: a 30% difference in rolling resistance can equate to a 5% increase in fuel consumption.
We tested three examples of the same new tyre, each with a different amount of air in it: one tyre was inflated to the recommended 36psi (2.5bar), the second at 22psi (1.5bar) and the third at 51psi (3.5bar).
The under-inflated tyre had a 4.5% higher rolling resistance than the correctly inflated tyre: 8.48kg/t compared with 6.67kg/t. This difference in equates to an additional £4.50 of fuel for every 100 miles if the tyres were fitted to an Audi A3 1.5 petrol. Over the course of a year and 10,000 miles that adds up to an extra £450 – enough to buy 112 takeaway coffees at £4 each.
The over-inflated tyre had a rolling resistance of 6.08kg/t, which is 1.5% higher than the correctly inflated one, making it only a little more costly for fuel. However, the safety implications of having under-inflated tyres are even more of a concern. A series of wet and dry track tests carried out by What Car? in an Audi A3 1.5 petrol demonstrate how dangerous it can be to drive with too little air in your tyres.
Driving with the under-inflated tyres, our test car took 1.3 metres more to come to a halt from 50mph in the wet. The tyres also lost grip on a wet road surface 5mph sooner than the correctly inflated tyres – this additional stopping distance means a car is far more likely to crash into another vehicle in front or hit a pedestrian.
Handling was also adversely affected when the test car was driven on tyres that were low on pressure. The car took 1.0sec longer to complete a circuit of the wet handling track with underinflated tyres than it did when they were correctly inflated, because they lost grip sooner, meaning the car had to be driven slower.
The final test conducted highlighted the serious threat to safety posed by having a single under-inflated tyre – in a real-world situation this replicates the car having a slow puncture on one tyre. The rear nearside tyre was deflated to 1 bar (22psi), and this upset the balance of the car so much that the car skidded so violently that it veered off the wet handling track onto the grass verge. This meant that the test driver wasn’t able to complete the required three laps.
What Car? consumer editor, Claire Evans, commented: “It’s good to know that you’ll save money on fuel if your car’s tyres aren’t under-inflated, but it’s the safety implications of driving with too little air in them that make the most compelling argument for checking tyre pressures regularly.
The tyre inflation tests were conducted as part of the 2025 What Car? tyre test, which pitted tyres from five different brands against each other in a series of track and lab tests. The best performing tyre was the Continental PremiumContact 7, and the lowest scoring was the Triangle EffexSport TH202. The Michelin Primacy 5 was commended for delivering the best economy.
“Our main tyre test also highlights the significant difference in the handling and braking abilities of different tyres. Poorer performing tyres can take up to three metres longer to stop than the best tyres, and that could be the difference between having a near miss or hitting another vehicle in an emergency situation,” added Claire.
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