Millions of drivers are wasting hundreds of pounds in fuel a year simply by not inflating their vehicle’s tyres correctly, according to a new study.

Mechanics at a garage in Wales checked the pressures of 100 vehicles in for a service over the period of one month and found that 72 per cent had tyres not inflated to the manufacturer recommended guidelines.

In fact, 22 per cent had all four tyres inflated at the wrong pressure.

Significantly, four in five cars and vans found to have incorrect pressure levels were underinflated – a mistake that can make a vehicle up to 10 per cent less fuel efficient and cost a driver of a family petrol car around £140 a year in wasted unleaded.

A review of 100 vehicles in for a service between September and October found that almost three quarters (72%) had incorrectly inflated tyres

The analysis was carried out by the garage as part of an investigation by breakdown provider, Green Flag.

It has estimated that the total cost of wasted fuel as a result of drivers having incorrectly inflated tyres – be it just one or all four – in the UK is a staggering £1.2billion a year.

For those motorists who were driving with all four tyres with pressure below the manufacturer recommended level, the impact can be significantly costly for fuel bills.

Philip Gomm, spokesman at road safety charity the RAC Foundation, explains: ‘Incorrectly inflated tyres can be bad for safety, but also have an impact on fuel efficiency. 

‘If they are under-inflated fuel economy is cut by as much as ten per cent because of the extra resistance with the road.’

For a petrol family car returning 40 miles per gallon (mpg), the annual fuel cost (based on the current price of unleaded at 150.69p, according to RAC Fuel Watch) to cover the UK average mileage of 7,400 miles per year is £1,267.

But with underinflated tyres reducing fuel efficiency by up to 10 per cent, slashing economy to 36mpg, would see an annual fuel bill rise to £1,407, based on This is Money’s calculations.

Experts warn that underinflated tyres can reduce a vehicle’s efficiency by up to 10%. For the average family car owner, this could mean £140 of fuel is being wasted annually

While underinflation of tyres can be caused by punctures, it is most commonly the result of negligence by an owner.

For owners of the latest models with tyre pressure sensors fitted to their cars, there is no excuse, as their dashboard will tell them if they need to pump more air into one or more tyres well below the approved level. 

Every car has a recommended tyre pressure measured in Pounds per Square Inch (PSI), which will vary depending on the make and model. 

This recommendation will be listed in the owner’s manual and common on the inside of a front door sill. 

Green Flag’s study – carried out between September and October this year – found that drivers are running their cars with pressures way off the guidelines.

For those tyres found to be underinflated, the PSI was below by an average of seven PSI. And for the tyres that were over inflated, the average was five PSI over the recommended pressure. 

Incredibly, some car tyres that were tested were found to be 20 PSI under the number recommended by the vehicle manufacturer – and others measured up to 11 PSI above the recommended level.

By driving with tyres so underinflated comes with its own risks.

A seriously underinflated tyre can result in overheating and the sidewall could blow out, causing a rapid deflation of the tyre that could cause the driver to lose control of the vehicle. Incorrect tyre pressures will also lead to irregular or premature wear on the tyre, meaning they will need to be replaced more frequently, Green Flag says.

And it’s not just the tyres fitted to the cars that are inflated incorrectly. 

A third (32 per cent) of the 100 vehicles in for a service had a spare tyre. 

However, just 28 per cent of these were incorrectly inflated. 

While spare wheels and tyres are almost disappearing from new models as a standard feature, those fortunate to have one are likely risking rendering this back-up solution redundant if the spare in the boot is not suitable to be fitted if the driver is unlucky enough to suffer a puncture.

The breakdown provider says motorists must make sure that all tyres – not just those on all four wheels – are maintained correctly so that they meet the legal standards in the event they are used.

Katie Lomas, managing director of Green Flag, commented: ‘Our study reveals a worryingly high proportion of Brits are driving on incorrectly inflated tyres. 

‘This can be incredibly dangerous and costly so it’s vital that drivers check their cars’ tyre pressure regularly, especially when the car is fully loaded or if they are planning on heading out on a long journey. 

‘Cars do naturally lose pressure over time so it is important to check them regularly to ensure that they are correctly inflated in line with the manufactures’ recommendation.’