Your roof box is more thirsty than you think

What Car? reveals big gap between official and its own, tested figures for roof box-laden fuel economy
Roof Box cars: how thirsty is yours?
What Car|Doug Kno14 October 2015

What Car? test results suggest that official fuel economy figures for cars carrying roof boxes may be seriously misleading.

The What Car? True MPG team road-tested two cars, the Dacia Logan and Nissan Qashqai – with and without roof boxes.

The team has already proved that the cars do not achieve their official figures without roof boxes. The Logan is 27% less efficient than quoted and the Qashqai comes to within 21% of its Government-approved mpg rating.

Fitting a 430-litre roof box with 50kg on board, however, brought those figures down even further: to 35% and 31% respectively. The Dacia managed 48.5mpg, compared with the officially stated 74.3mpg, and the Qashqai achieved 32.1mpg (officially 47.1mpg).

What Car? editor Jim Holder said: “The vast majority of cars are unable to match the official mpg figures that manufacturers quote even before the addition of a roof box. Adding the box increases the overall weight of the car and increases aerodynamic drag, so it stands to reason that your fuel efficiency will be harmed.”

He added that while taking a UK break might be considered a good way to save money “you’re effectively paying for it in other, less obvious ways”.

Over 600 vehicles have been assessed by What Car? to check their real-world efficiency. It has found that economy figures are on average 20% lower than those quoted by the Government, which are worked out in lab tests.

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