Dear Car Talk:
I have a question for your MIT engineering team.
Many people where I live in California leave their empty rooftop bike/surfboard/kayak racks on their cars all the time. They're either too lazy to remove them, they think they might use them again soon or they just like the coolness vibe these carriers give off.
What is the fuel economy hit for doing this? Even more important to us here in the Golden State: When we're not carrying anything and we don't take down those mounting bars and carriers, how many more grams of pollutants are we spewing per mile? Are we looking at a 1% or a 10% effect?
What about empty behind-the-SUV bike carriers mounted into the trailer hitch? Are those as bad? Or is it significantly less of an impact to be behind the vehicle rather than on top? Thanks for any solid information. -- Cliff
Well, we didn't have to go to our lab at MIT to answer this question, Cliff. Consumer Reports did some testing and has the answer.
They found the fuel economy loss is as low as 2% and as high as 19%, depending on the type of car and type of equipment carrier.
Sedans are naturally more fuel efficient than SUVs, due to their shape and lower stance. And when CR put just a roof rack on a Nissan Altima traveling at 65 mph, it cost the Altima an 11% fuel economy penalty. When CR added a cargo carrier to the roof rack, mileage dropped 19%!
They also tested a 2019 Toyota RAV4. Since SUVs are already shaped like refrigerators, fuel economy is worse to start out with but dropped less during testing. The RAV4 saw only a 2% drop from the roof rack alone and a 13% drop when the cargo pod was added. The bike rack that attaches to a tow hook behind the car did less damage to the car's fuel economy, averaging a 2%-3% loss when not carrying bikes.
While they didn't test the corresponding increase in pollution these rack and carriers cause, you can assume that pollution is roughly proportional to fuel use. So, for the purposes of shaming your Sierra Club member neighbor, I think you can use those same numbers.
Needless to say, we recommend removing your surfboard, bike and Bactrian camel carriers when they're not in use. If you're afraid your outdoorsy image will take a hit, go old school and slap on some bumper stickers.