Roof boxes, roof racks and luggage: how much more fuel

Updated 23 September 2026

Loading the car for the holidays has two costs: weight, which shows mostly in town, and the aerodynamics of whatever sits on the roof, which soars on the motorway.

Roof boxes: mostly aerodynamics

According to the US Department of Energy, a large, blunt roof box reduces fuel economy by 2–8% in city driving, 6–17% on the highway and 10–25% at 65–75 mph. German motoring club ADAC tested ten roof boxes on a VW Touran diesel at 130 km/h: consumption rose by 0.94 to 1.29 L/100 km, 18% on average. On a compact sedan, ORNL measured penalties of 25–27% at high speed.

Increase in fuel consumption with a roof box
Road type	Value used	Range
Town	+4%	2–8%
Country road	+10%	6–17%
Motorway (≈ 120 km/h / 75 mph)	+17%	10–25%

The calculator also adds about 50 kg of weight (box and contents).

Empty bars and rear-mounted carriers

A study by Lawrence Berkeley National Laboratory and NREL estimated that roof racks account for 0.8% of all US light-duty vehicle fuel use, and that most of it comes from empty racks driven on highways: removing them when not needed is the easiest saving.

Rear-mounted carriers on a tow hitch cost far less: 1–2% in town and 1–5% on the highway, according to the DOE.

Weight: passengers and luggage

According to fueleconomy.gov, every extra 100 lb (45 kg) reduces fuel economy by about 1%, and the effect is larger the lighter the car. The ICCT sums up the literature: 10% more mass raises consumption by 4–5% if the engine is unchanged. Weight matters more in town, with frequent acceleration and braking, than on motorways, where aerodynamics dominates.

Increase in fuel consumption per extra 100 kg (mixed driving)
Car type	Approximate mass	+100 kg (220 lb)
Small	1,150 kg	≈ +3.1%
Compact	1,400 kg	≈ +2.5%
Saloon	1,600 kg	≈ +2.2%
SUV	1,800 kg	≈ +2.0%

Values from the calculator’s model with a known real-world figure. With a WLTP figure, the first 100 kg are already included in the official test.

How to reduce the impact

Put heavy items inside the car and keep the roof box for bulky, light things.
Take the box and bars off as soon as you no longer need them.
If you can, use a rear-mounted carrier instead of a roof one.
With a roof box, slow down: ADAC advises not exceeding 130 km/h (80 mph).