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Will a roof box increase my car's fuel consumption?

Your motoring problems solved


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I need to buy a roof box for my 2003 Audi RS 6 to accommodate my family’s expanding paraphernalia. I regularly travel in France and Germany and am concerned that a roof box might increase my fuel consumption and mean that I have to drive more slowly. AB, London SW14

Adding a roof box means your car has to carve a bigger hole in the air, which requires burning more fuel, so your mpg will suffer. It’s impossible to give an exact figure because it depends on so many factors, but Britain’s biggest roof-box manufacturer, Thule, states that your mpg is unlikely to decrease by more than 5%. If fuel consumption is a concern, invest in a streamlined box, which will cut through the air more efficiently. The downside is that the shape is less convenient for packing bulky items. Take a look at roofbox.co.uk or halfords.com to see the options.


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Make sure that when full the box doesn’t exceed the maximum limit for your car, usually 50kg or 75kg including the weight of the container itself. Just as important, always load the box evenly and use the internal tether straps to keep the contents in place. Whenever you stop, check that the box and its load remain secured.

The maximum speed permitted on French roads is 130kph — about 81mph — and most popular roof boxes are designed to cope with that speed. On some German autobahns, where there is no speed limit, a good roof box should allow you to drive up to about 90mph, depending on the conditions, according to Thule. A spokesman for the company said it would not recommend travelling faster than that because of the impact of crosswinds on the larger side profile, which could make the car difficult to manoeuvre and potentially unstable. 

Sunday Times Driving car clinic expert: Dave Pollard

INSPECTOR GADGET
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory – read more from Dave here.

GOT A PROBLEM?
Email your question to carclinic@sunday-times.co.uk or write to Car Clinic, Driving, The Sunday Times, 3 Thomas More Square, London E98 1ST, with a daytime phone number, your address and as much detail about your car as possible. We can’t reply in person, so don’t send original documents or SAEs. Advice is given without legal responsibility.