The common item that adds '39%' drag with petrol and diesel vehicles to use more fuel

Petrol and diesel drivers risk increasing drag and wasting fuel by leaving a common item stuck to their vehicles. Experts at the RAC have urged road users to never leave a roof rack or roof bar on their vehicles if they are not being used.

Forgetting to remove the tools may appear harmless but can actually have a major impact on fuel use. Leaving an empty roof box can have a major effect on wind resistance, increasing drag and impacting the efficiency of vehicles. It means motorists will likely use up more petrol and diesel than would be necessary in a major blow to cash-strapped road users.

The RAC explained: “Don’t leave your roof bars and roof box on because they create wind resistance and cause your car to use more fuel through the ‘drag’ effect. This is increased the faster you drive.

“According to the Energy Saving Trust an empty roof rack adds 16% drag when driving at 75mph. At the same speed, a roof box adds 39%, making your vehicle much less fuel efficient.”

Analysis from RAC Fuel Watch shows that unleaded petrol is currently priced at 132.75p per litre, the lowest level since 2021.

Meanwhile, diesel is the most affordable it has been since 2024 with fees currently standing at 139.03p.

Experts at Halfords have stressed the extra drag will force engines to “work harder” to maintain speed.

However, they suggested that the actual impact on fuel use is likely to be less than many would expect.

Halfords explained: “Thanks to the changes in aerodynamics that roof boxes cause, you will see a slight increase in fuel usage if you use a roof box.

"As well as the extra weight of both the box itself and the items inside it, you'll also be creating extra drag – also known as air resistance – which will force your engine to work harder to maintain speed.

“However, that increase will be minimal. Thule, one of the biggest manufacturers on the market, estimate that carrying a roof box only leads to a 5% increase in fuel consumption, so you won't be stopping more regularly to fill up.”