SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/cars-roof-cargo-box-mpg

Road trip! A big rooftop cargo box at Interstate speeds can cut your gas mileage by up to about how much?

Up to about 25% at 65 to 75 mph, because of wind drag.

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  1. About 1%, it's barely noticeable
  2. It improves mileage by smoothing airflow
  3. Up to about 25% ✔

Why: A blunt box on the roof pushes against the wind. The Department of Energy says a large one cuts mileage around 10% to 25% at Interstate speeds. Take it off when you don't need it.

Huh, didn't know that: A rear-mount cargo box or tray costs much less mileage than a rooftop box.

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Why did the car bring a suitcase? Because it was ready for a long trip and didn't want to be caught short!
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References

Answer: Up to about 25%. A blunt box on the roof pushes against the wind. The Department of Energy says a large one cuts mileage around 10% to 25% at Interstate speeds. Take it off when you don't need it.

  1. Driving More Efficiently (fueleconomy.gov, US DOE) www.fueleconomy.gov/feg/driveHabits.jsp
    6% to 17% on the highway, and 10% to 25% at Interstate speeds (65 mph to 75 mph)
    Checked 2026-10-11.

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