How Much Does a Roof Box Cost in Fuel?
November 8, 2026 · 9 min read
The box is on the roof and someone has just asked what it is doing to your gas bill. Here is the number. Roof box fuel economy losses show up clearly in published testing, and they come from air resistance rather than weight — which is why the answer swings so much with speed.
Updated — first published
Quick answer
A loaded roof box cuts fuel economy by roughly 10% to 25% at interstate speeds of 65 to 75 mph, which on a 600-mile trip in a car that normally returns 30 mpg costs about 3 to 5 extra gallons. The penalty comes from air resistance, so driving 55 mph instead of 75 mph cuts it roughly in half.
Who this guide is for
This is for you if you own a cargo box (also called a roof box) and want a dollar figure for the drive ahead. You need two numbers: your highway mpg and the trip distance.
This is for you if
- You are driving 300 miles or more with a loaded box on the roof
- You are deciding whether to leave the box on between trips
Look elsewhere if
- You are towing a trailer, where the penalty is much larger
- You drive an electric car, where the loss shows up as range
How this guide was researched
Every figure comes from published vehicle testing, with the arithmetic shown so you can redo it for your own car. Cedar Buy ran no tests of its own.
How this guide was researched
- Read in November 2026: Department of Energy guidance on fueleconomy.gov; Oak Ridge National Laboratory testing published as SAE 2014-01-1614; Chen and Meier in Energy Policy volume 92 (2016); Consumer Reports testing of a 2019 Nissan Altima and a 2019 Toyota RAV4.
- Results measured at a stated steady speed counted most: a percentage with no speed attached cannot be checked.
- Left out: drag claims with no published test method, and forum mpg reports. The costs below are calculated from the published ranges, not measured on any box.
- Specifications come from manufacturer documentation. Cedar Buy does not physically test products — see our editorial policy.
The short answer, in gallons and dollars
On a 600-mile trip at 65 mph, a loaded roof box costs a 30 mpg car about 3.3 extra gallons. The Department of Energy puts the loss from a large, blunt rooftop cargo box at 6% to 17% on the highway, and 10% to 25% at interstate speeds of 65 to 75 mph. The table turns the middle of those ranges into fuel.
| Trip length | Steady 55 mph | Steady 65 mph | Steady 75 mph |
|---|---|---|---|
| 200 miles | 0.6 gal, about $2 | 1.1 gal, about $4 | 1.7 gal, about $7 |
| 600 miles | 1.7 gal, about $7 | 3.3 gal, about $13 | 5.0 gal, about $20 |
| 1,200 miles | 3.5 gal, about $14 | 6.5 gal, about $26 | 10.0 gal, about $40 |
What the table does not show
Put in your own mpg and pump price and the shape holds. A crosswind, a mountain pass or a tall box on a small sedan all push the number higher: this is the typical case, not a ceiling.
What published tests measured
The clearest numbers come from Oak Ridge National Laboratory, which drove a 2009 Toyota Corolla and a 2009 Ford Explorer carrying 100 pounds in a roof box. At 65 mph the Corolla lost 25% of its fuel economy and the Explorer 10%, while the same 100 pounds on a rear hitch tray cost each about 1%.
Consumer Reports found the same on newer cars: at 65 mph a 2019 Nissan Altima lost 19% with a roof rack and carrier fitted, and a 2019 Toyota RAV4 lost 13%.
Empty box vs loaded box
An empty box costs nearly as much as a full one, because you pay for the shape, not the contents: in the Consumer Reports testing the bare rack alone cost the Altima 11% before any carrier went on top.
Weight is the part people overestimate: the same 100 pounds cost about 1% on a rear tray, so packing light saves little. Leaving the box at home saves the lot.
Bulky, light gear is the honest case for a box: skis and snowboards are long and hard to fit inside a sedan. Our guide to the best ski and snowboard cargo boxes (2026) covers the shapes built for that.
Speed is the biggest factor
Speed changes the bill more than anything else you control, because air resistance rises with the square of speed: double the speed and the air pushes back four times as hard. A box that costs 8% at 55 mph costs around 20% at 75 mph.
Over 600 miles, 65 mph instead of 75 mph saves about 1.7 gallons and costs a little over an hour.
Box shape and the difference it makes
Shape beats capacity: for the same gear, a long, low box costs less fuel than a short, tall one, because it adds less height above the roof and lets the air close behind it cleanly. Shape is also what separates one box from another once fit is settled, which is where our best roof cargo boxes (2026) roundup starts.
How much height it adds
Height above the roofline drives the drag, because it sets how much extra air the car pushes aside. Of two boxes holding the same gear, take the longer, lower one.
Where it sits on the roof
Mount it as far back as your crossbars and hatch allow, where it is out of the air coming off the windshield.
The crossbars underneath
Crossbars are the two bars the box clamps onto, and square ones drag more than flat aero bars. A bare rack took 11% off the Altima on its own, so take the bars off too.
The drag figure in the marketing
Ignore a drag coefficient quoted with no test method and no vehicle named: a figure from one car says nothing reliable about the same box on yours.
Taking it off between trips
Take the box off when the trip ends, and the crossbars with it. This is the cheapest saving here, because the drag is there on every commute whether the box carries anything or not. Chen and Meier, estimating in Energy Policy volume 92 (2016) that roof racks burned about 100 million extra gallons of gasoline in the US in 2015, found that total turned mostly on cars wearing empty racks.
The trade-off is real: the box and bars take 10 to 20 minutes to remove and need somewhere dry to live. Use a one-week rule — unused for longer, the fuel costs more than the time.
Working out your own cost per trip
You need three numbers and one minute. Divide trip miles by your highway mpg for the gallons you would burn without the box, then multiply by 0.09 for 55 mph, 0.16 for 65 mph or 0.25 for 75 mph. That is the extra fuel; in a small sedan, use the next multiplier up.
Worked through: 450 miles at 24 mpg is 18.75 gallons; times 0.16 is 3.0 extra gallons at a steady 65 mph. Multiply by what you pay — pump prices move weekly, and the Energy Information Administration publishes the national average — so at $4.00 a gallon the box costs about $12.
Key terms, explained
Three terms do most of the work here, and two get used loosely.
Plain-language definitions
- Frontal area
- The size of the hole a vehicle punches through the air, measured head-on in square feet. A box adds to it directly, which is why height matters more than length.
- Drag coefficient, written Cd
- A number for how cleanly a shape slips through the air, around 0.30 for a modern sedan. Drag is the coefficient multiplied by frontal area, so a slippery shape that is tall still costs fuel.
- Coastdown test
- The standard way drag is measured on a real vehicle: the car is run up to speed on a flat track, then left to slow with the engine disengaged. The method is set out in SAE J1263 and SAE J2263.
Mistakes that cost people money
The expensive mistake is not picking the wrong box. It is leaving the right one on for eleven months of the year.
- Leaving the box on all yearA box fitted in June and forgotten until next summer burns fuel on every school run, and an empty rack alone measured 11% at 65 mph.
- Packing light to save fuelWeight is the smallest part of it: 100 pounds cost about 1% on a rear tray in the Oak Ridge tests, against 25% in a box on that car’s roof.
- Buying a used box without checking the fitA box that does not match your crossbar spacing ends up too far forward, the worst position for drag. Our guide to buying a used cargo box: what to check lists the measurements.
Frequently asked questions
How much mpg do you lose with a roof box?
Expect to lose 6% to 17% on the highway and 10% to 25% at 65 to 75 mph, according to Department of Energy figures. At 65 mph in Oak Ridge testing, a Toyota Corolla lost 25% with a loaded box and a Ford Explorer lost 10%.
Does an empty roof box still use more gas?
Yes, almost as much as a full one, because the penalty is drag from the shape rather than the weight inside. In Consumer Reports testing at 65 mph, a bare roof rack cost a Nissan Altima 11% of its fuel economy, and the carrier on top took that to 19%.
Is it worth taking the roof box off between trips?
Yes, if it will sit unused for more than a week. Removing the box and crossbars takes 10 to 20 minutes and clears the drag from every mile in between. Researchers estimating the national fuel cost of roof racks found empty racks were the largest contributor.
Does a roof box hurt a small car more than an SUV?
Yes. A box adds proportionally more height and drag to a low car than to a tall one that already pushes plenty of air. At 65 mph with the same 100-pound load, Oak Ridge measured a 25% loss on a 2009 Toyota Corolla against 10% on a 2009 Ford Explorer.
What to do before your next trip
Do the sum for your own car, then act in this order: if the gear fits inside, leave the box at home; if it does not, set the cruise at 65 mph; take the box and crossbars off the day you get back. The last step keeps paying all year.
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