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.
Roof box fuel consumption.
But no matter how aerodynamic it might look it also alters the flow of air around your carefully shaped wind tunnel tested.
He found that compared to the baseline of no roof rack the empty rack reduced his mpg by 12 7 and the roof rack with a bicycle reduced it by a whooping 27 3.
Why do they have such an effect.
Roof boxes reduce mpg.
He compared the gas mileage of his car at 54 7 mph 88 kph with 1 no roof racks 2 an empty roof rack and 3 a mountain bike on the roof rack.
Their study was published recently in the.
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.
They found that in 2015 roof racks nationwide were responsible for 0 8 percent of light duty vehicle fuel consumption or 100 million gallons of gasoline.
Couldnt fit into any car parks with a height restriction or multi stories.
However with the roof box it was only capable of 32 5mpg 31 per cent less than its claimed economy via the government tests.
Leaving engines aside for a moment two major elements affecting a car s economy are weight and aerodynamics.
6 0 l 100 km 39 mpg thus the cargo box on the roof increased fuel consumption by about 5 compared to the baseline.
5 9 l 100 km 40 mpg roof mounted thule xtender 739 ski carrier with skis.
I would guess a roofbox adds around 5 to fuel consumption although there are many factors at play especially if you if you sloiw down to 60 65 you may then even see a reduction in fuel compsumption.
Normally car does 33 mpg down to about 30 just now service due.
The gear with the most impact were the dual mountain bikes strapped to the roof which impacted the fuel economy by 31 percent or 9 mpg.
A close second was the kayak with 28 percent or a.
5 6 l 100 km 42 mpg roof rack with thule dynamic 800 aerodynamic cargo box.
Last year on a run to paignton 900 round trip we got about 35mpg fully loaded roof box included.
