Car Talk: Improving your car’s aerodynamics will always improve mileage

Dear Car Talk:

I removed the cross bars on my 2019 Toyota RAV4. My beloved queried, “What for?” I said to eliminate the jet propulsion din if we open the moonroof AND to get better mileage.

“How much better?” she asked.

So, now I need your help. Will I actually get better mileage, or am I full of bologna? — Paul

P.S.: I miss your brother and his laugh. My brother and I miss our younger brother, especially his laughter.

Thanks, Paul, and condolences on your brother. I don’t know whether you’re full of bologna. But I’m sure your beloved has done extensive research on that, so ask her.

As for your car, improving your aerodynamics will absolutely improve your mileage.

Car makers know that anything that “catches” air adds noise and reduces fuel economy. Basically, the harder your car has to work to push through the air, the more fuel it has to use.

So engineers and designers go to great pains and enormous expense, using computer models and wind tunnels, to make sure that air slips easily and smoothly over and around your car.

And then we — the buying public — take these carefully designed aerodynamic cars home and stick our kayaks on top of them.

I’m sure it makes the engineers cry when they pass us on the road, Paul.

Even without the kayak, the roof rack itself makes your car less efficient. One study, from Berkeley Labs and the National Renewable Energy Laboratory in 2015, using crowd sourced data, reported that the loss from a roof rack alone can be up to 25%. That sounds high to me.

Consumer Reports, which does extensive fuel economy testing, found that the loss from an empty roof rack was between 2% and 11%. The sedan they tested, a 2019 Nissan Altima, saw its highway mileage drop 11% when driven with an empty roof rack.

The other vehicle, which happens to be just like yours, Paul — a 2019 RAV4 — saw its mileage drop about 2% with a roof rack. Consumer Reports speculates that the RAV4, as an SUV, was already a boxier and less efficient shape than the sedan, so the impact of the roof rack was lower.

It’s like when I broke my nose, it didn’t make me look much uglier, given where I was starting from.

So the answer, for your car, is 2%, Paul. And the answer for everyone else out there driving with a roof rack, or removable cross bars, is to take them off when you don’t need them. Or, rent a kayak when you get there.

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