Car Talk

January 2006

Dear Tom and Ray:

I'm wondering if putting a roof rack on a car will decrease its gas mileage. Our science teacher gives weekly extra credit, and to be frank, I NEED the extra credit. She said that I should e-mail you to ask this. Please reply soon, and be aware that there may be others asking this week! Thanks, and I'm sure that my parents will thank you someday for helping me keep my grades up! -- Phil

Beat up old car

TOM: Well, to be fair to all of the kids in your class, Phil, we're printing our answer here in the newspaper instead of e-mailing it to you. That way, the advantage will go to those students who actually take the time to look at a newspaper occasionally (not a bad idea for those of you junior citizens who will end up running the country in four or five decades).

RAY: The answer is yes, a roof rack will decrease your mileage. There's a field of science called aerodynamics, which studies how air flows around stuff. In general, the smoother an object is, the more easily it moves through the air. This is why you see very few roof racks on airplanes.

TOM: Now, there are lots of strange things that air currents do. They swirl and create eddies and pockets of turbulence. So wind resistance is not entirely predictable by just looking at something's shape. But generally speaking, the smoother or sleeker a surface, the less wind resistance it creates. And the less resistance it creates, the less energy (i.e., gasoline) you need to push it through the air.

RAY: So, because the top of a car is generally flat and smooth, and a roof rack is not, the roof rack adds additional resistance to the car's shape and decreases your mileage.

TOM: Now, for extra credit, Phil, a two-part question: (A) Does removing the cross members of the roof rack (when it's not in use) help increase your mileage? And (B) Why does it or doesn't it?

Back to the January 2006 index

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