Do Roof Racks Affect Gas Mileage?

Consumer Reports tested a sedan and an SUV with and without roof gear. On the sedan, adding just the roof rack dropped fuel economy by almost 11%, about 5 mpg. Adding a rooftop carrier on top of the rack took the total loss to 19%, about 9 mpg.

On the SUV the pattern was the same but the percentages were smaller: the empty rack cost nearly 2%, and the rack plus carrier cost 13%, just over 5 mpg.

Car and Driver separately measured a 12% efficiency drop from crossbars alone.

Two things to take from that table. The carrier costs far more than the rack, which is obvious. And the slippery car loses a bigger percentage than the boxy one, which is not — a sedan is efficient precisely because its airflow is clean, and a box on the roof destroys that advantage in a way it cannot on a vehicle that was never slippery.

Aerodynamic drag scales with the square of speed. Double your speed and drag quadruples. At town speeds the engine spends most of its effort on rolling resistance and acceleration; at 70 mph it spends most of it pushing air aside.

A roof box does three things at once, all bad. It adds frontal area. It disturbs the airflow along the roof, which is where a car’s carefully designed attached flow lives. And it creates a turbulent wake that the rest of the car then has to drag along with it.

An empty rack does the last two without the first, which is why a bare set of crossbars still costs you real money. There is no such thing as a free rack left on the car.

1. Take the rack off between trips. The single largest saving available to most people. An empty rack you use four times a year costs you fuel for 361 days.
2. Use the hitch instead of the roof. A hitch carrier sits in the low-pressure wake behind the car rather than in clean air above it. The penalty is far smaller.
3. Choose a low-profile box. Less frontal area is less drag, directly.
4. Mount the box as far back as the bars allow. Air that has already been disturbed by the windshield costs less to disturb again.
5. Slow down. Because drag is a square law, dropping from 75 to 65 mph is a disproportionately large saving with a loaded roof.
6. Empty the box before you put the car away. Weight matters far less than shape, but there is no reason to carry it.

An EV at highway speed is spending almost all of its energy on aerodynamic drag — there is no idling loss, no gearbox loss, and regenerative braking recovers much of the rest. Which means a roof box takes a bigger proportional bite out of an EV’s range than out of a combustion car’s fuel tank.

It also matters more practically. Losing 15% of a fuel tank means one extra stop at a station that takes five minutes. Losing 15% of range means re-planning where you charge.

We have not measured this and will not publish a figure we did not produce. Drive the first loaded leg watching consumption, and plan the rest of the trip on that real number rather than the EPA one. Our Model Y page goes into the specifics.