How much do roof racks hurt gas mileage?
I'm going on a big national car trip, and I want to increase cargo space by getting a roof rack, but I'm wondering how much the additional drag will increase my gas bill. Is there any way to figure this out?
Please remember that there are two different mpg's: US and UK. mpg is "miles per gallon" - unfortunately there are two different gallon sizes used, a US gallon of about 3.78 litres and an UK gallon ("imperial gallon") of 4.54 litres. As a result, the same car that does 50 mpg in the USA does 60 mpg in the UK. Fuel consumption is made up of several components: Losses in the engine, rolling resistance, and wind resistance. The roof rack will mostly increase the wind resistance, and wind resistance grows enormously with speed, much more than rolling resistance. So a slight reduction in speed will
This greatly depents on the car. As shown in another answer with a small car (with a small engine) it will greatly affect fuel usage (but i'm shocked on the 50%). I've recently fitted a roofrack on my jeep and hardly noticed the difference. This is partly because of the brick like aerodynamics and because of large diesel inside. The only answer I can give is that depending on your type of car your milage can change by as much as 50% or as little as 2%.
A local newspaper here in Finland recently ran a fuel efficiency test that might provide some relevant data here. The original article is, alas, in Finnish only, but I'll summarize the results below.
The test compared the fuel consumption of the test vehicle (a Volvo V70 D4 diesel), on a test route that included both highway and urban driving, in three rooftop configurations. The results were:
- No roof rack: 5.6 l / 100 km (≈ 42 mpg)
- Roof rack with Thule Dynamic 800 aerodynamic cargo box: 5.9 l / 100 km (≈ 40 mpg)
- Roof mounted Thule Xtender 739 ski carrier with skis: 6.0 l / 100 km (≈ 39 mpg)
Thus, the cargo box on the roof increased fuel consumption by about 5% compared to the baseline.
(The full test also measured the noise level at the driver's ear, and compared the ease of installation and various other aspects of the different options, but I'll leave those out of this summary. Suffice to say that the cargo box caused some steady but noticeable hum, and made the car somewhat more sensitive to side winds.)
It's billed as an aerodynamic box, and if that figure is correct then it certainly is just that - only 5% effect is pretty great! This would seem to indicate that the design of the box - or lack there of - is very important in determining the effect on gas mileage.