Tyres under-inflated by 15psi can raise fuel consumption by up to 6% - roughly £75 a year for an average driver. It is the cheapest efficiency fix there is: five minutes with a forecourt air line, no parts, no garage. At the current average of 161.2p a litre, a driver covering 8,000 miles at 40mpg spends about £1,464 on fuel, so 6% is £88.
What under-inflation costs
| Annual mileage | Fuel cost at 40mpg | +3% (mild) | +6% (15psi low) |
|---|---|---|---|
| 5,000 | £915 | £27 | £55 |
| 8,000 | £1,464 | £44 | £88 |
| 12,000 | £2,196 | £66 | £132 |
| 20,000 | £3,660 | £110 | £220 |
The RAC puts the figure at up to 6% for tyres 15psi below specification. Even a few psi low costs 2% to 3%.
Why it happens
An under-inflated tyre flattens where it meets the road. That larger contact patch flexes more as the wheel turns, and flexing rubber turns energy into heat. The engine has to replace that energy, which means more fuel. It is the same reason a bicycle with soft tyres is harder work.
Tyres lose pressure naturally - roughly 1 to 2psi a month, faster in cold weather. A tyre checked six months ago is almost certainly low now.
How to check properly
- Check cold. Driving heats the air and raises the reading. Check before a journey, or after the car has stood for a couple of hours.
- Find the right pressure. It is on a sticker inside the driver's door jamb, inside the fuel filler flap, or in the handbook - not on the tyre itself, which shows the maximum the tyre can take.
- Use the loaded figure when loaded. Most cars list a higher pressure for a full car or heavy luggage. Holiday trips are exactly when people forget.
- Do not forget the spare. If you have one.
- Check monthly. While you are filling up, since you are already there.
Over-inflation is not the answer
Pumping tyres above the recommended pressure does reduce rolling resistance slightly, but it shrinks the contact patch, which reduces grip and lengthens braking distances. It also wears the centre of the tread prematurely. The manufacturer's figure is a balance of economy, grip, comfort and wear - a few psi over is tolerable, significantly over is not worth the risk.
It is not just fuel
Under-inflated tyres build heat, which is the main cause of high-speed blowouts. They also wear unevenly on the outer shoulders, shortening tyre life. And tyres below 1.6mm tread across the central three quarters are an MOT failure and carry a fine and penalty points per tyre. You can check any vehicle's MOT history with our free MOT checker.
A year of rising prices made it worse
Under-inflation is a percentage penalty on a price that has just gone up sharply. Twelve months ago the national average petrol price was about 134.0p a litre; it is now 161.2p. The 27.2p rise did not make your tyres any softer, but it raised the price of every litre the soft ones waste - the same 6% consumption penalty costs about a fifth more in pounds than it did last autumn.
Framed the other way round: driving all year on properly inflated tyres instead of under-inflated ones saves roughly as much fuel as the first four months of this year's price rises added. It is one of the few maintenance tasks that pays directly at the pump, and the forecourt habit stacks on top of it - a 10.9p-a-litre town spread and correct pressures are not either/or. At diesel prices the arithmetic is steeper still: the same 6% penalty against a 182.4p average wastes measurably more per mile than it did against any diesel price in our national series before this year, so the case for a monthly pressure check has been strengthening with the pump price itself.
How this compares with other savings
Correct tyre pressure saves up to 6%. Driving more smoothly saves 10% to 15%. Filling at a station 10p cheaper saves about 6% of your fuel bill outright, for thirty seconds of checking. All three stack - see how to find cheaper fuel near you and our fuel cost calculator.
Frequently asked questions
How often should I check?
Monthly, and before any long journey. Tyres lose 1 to 2psi a month naturally, so quarterly checking is not enough.
Do forecourt air machines charge?
Many do, typically 50p to £1, though some supermarkets provide it free. It pays for itself immediately if your tyres are low.
Does cold weather affect pressure?
Yes. Pressure falls roughly 1 to 2psi for every 10°C drop. Tyres set correctly in summer will read low in winter, which is why a check as the weather turns is worthwhile.
Are the dashboard warnings enough?
No. Most tyre pressure monitoring systems only warn at around 25% below specification - by which point you have been losing fuel and wearing tyres for some time. Treat the warning as a failure alert, not a maintenance schedule.
Is fuel the only cost of running soft?
No - it is not even the main one. Under-inflated tyres wear their shoulders faster, handle worse and run hotter, and the safety margin in an emergency stop is the expensive thing to lose. Fuel is simply the cost you can measure weekly; the others show up as an early tyre bill or, at worst, a longer stopping distance.
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