Here you can find out how much of an impact a roof box has on your fuel consumption

Chrysler recently introduced a new version of the 2025 Chrysler Pacifica. This new family adventure vehicle comes standard with a 16 cubic foot Thule cargo box on the roof, which is a good deal. Rooftop storage is extremely useful on extended vacations and can be a lifesaver on overland or off-road adventures.

Luckily, you don’t have to buy a new Chrysler to get a nice roof box or roof rack. Depending on the make and model, you can purchase a set of roof rails for a hundred dollars, and it’s not hard to find a good deal on a new or used roof cabin. However, these add-ons come with their own hidden costs. This means they actually reduce your fuel consumption if you leave them on the roof of your car between trips. Consumer Reports published a fairly informative study on this issue a few years ago, and its findings are as relevant today as ever.

How much is this $200 roof rack really costing you?

Consumer Reports conducted a test with a 2019 Toyota RAV4 and a 2019 Nissan Altima and tested the vehicles with a roof rack as well as with a roof rack and a roof cab. Perhaps the most surprising finding of the study was that the sedan actually took a big hit in fuel efficiency just because of the roof rack.

Your fuel economy is less affected when you drive at slow speeds. It’s the highway driving that really ruins your mileage. Once you get to the campsite and are ready for some slow, low rock climbing, wind resistance is no longer a real problem. Therefore, these tests were conducted based on an average speed of 65 miles per hour.

The Altima used in the test was capable of achieving up to 48 miles per gallon with no roof at all. It lost 5 mpg with the roof rack alone and a whopping 9 mpg with the roof bin attached.

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Consumer Reports conducted a similar study earlier this year using the same brands and models and found similar results. An empty bike rack cost the RAV4 2% of its fuel consumption, while two bikes cost the SUV 12% of its fuel consumption. The Altima suffered a more significant loss of 3% with an empty trailer hitch rack, or 25% with two bikes.

The added weight and compromised aerodynamics that come with installing large storage compartments on the outside of the vehicle naturally result in poorer fuel efficiency. These studies only give us more accurate numbers for what we have always known.

A more conservative estimate from the Department of Energy suggests an average fuel economy loss of about 5% when toting a loaded roof rack. On a broad average, this would cover large and midsize SUVs and pickups, as well as sedans and small SUVs. Bottom line: You’ll lose less fuel on larger vehicles like a Ford Expedition or a Chevy Tahoe, but every vehicle loses some level of efficiency when you pack the roof.

Roof pods could cost you around $1,000 after five years

Say you drive something like the 2020 Toyota RAV4 and get 30 miles per gallon. According to the EPA, you’ll need to spend about $1,500 per year over 15,000 miles per year. After five years that’s $7,500. If you spend an additional 13% at the pump on your roof rack and storage box, that comes to $975 after five years. For a roof rack alone, that 2% impact will cost you about $150, which isn’t that bad, but you’re still roughly doubling the total cost of ownership for the rack.

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For sedans, the numbers are about the same. Consumer Reports’ In the Altima test, the vehicle’s fuel efficiency decreased by 19% when it had a roof cab. The EPA estimates that a 2019 Altima with a 2.5-liter 4-cylinder engine gets about 32 miles per gallon, which equates to a fuel cost of $1,000 per year. After five years, fuel costs for a typical midsize sedan like an Altima, Camry or Accord are around $950, which is almost a whole additional year of gas just to transport the roof container.

Are small trailers a viable alternative to roof storage?

Short answer: RVs, regardless of size and weight, are even worse for your fuel economy than rooftop storage.

Exactly how much fuel consumption you lose when towing a trailer depends heavily on the weight, the aerodynamics of your trailer, and the horsepower rating of your vehicle. A good rule of thumb that most poles follow is to assume a 2% loss in fuel economy for every 100 pounds you tow.

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If we assume you’re towing one of these small teardrop trailers, the lightest of them typically weighs around 500 pounds, which is a 10% drop in fuel efficiency without taking wind resistance into account. Droplet trailer manufacturers report up to 20% fuel savings when towing one of their trailers. The brand’s flagship model weighs around 450 kilograms and is specially designed for use in smaller vehicles. Still, based on the brand’s own estimates, we think we’ll spend more towing this trailer empty than we would to carry a roof cab.

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That said, no, towing an extra half ton of cargo behind your truck, car, or SUV will not reduce your fuel economy.

Electric vehicle developers are doing some exciting things with self-propelled trailers that have no impact on mileage. However, currently these are prohibitively expensive and not really intended to be a practical load management system for the average off-road enthusiast.

Simply remove the storage system when you don’t need it

The solution to this problem is very simple: simply remove the racks from the roof when you don’t need them. We know it’s a pain in the neck. You’ve just gotten home from a long camping trip and all you want is a hot bath and a good night’s sleep in a real bed. You don’t want to be fiddling around with a screwdriver in the garage, and even an easy-release system seems like too much hard work.

We don’t really need to tell you to keep your car clean if you want to reduce fuel consumption, do we? Remove and put away all fishing gear from the back, remove the roof racks and cargo pod, empty the coolers, put the leftover sodas and beers back in the fridge, and stow all the camping gear. This is a basic common sense thing. We put it off not because we don’t know better, but because we’re too tired to do anything after a long trip.

That’s no problem. Just make sure your car is disassembled and cleaned up before you next hit the highway.

Sources:
Department of Energy, EPA, Droplet, Consumer Reports