Car roof racks have become increasingly popular in recent years, and that trend is forecast to continue: According to ShelfTrend, the global market for roof racks is expected to nearly double in size from 2024 to 2033, growing from about $2.3 billion to roughly $4.2 billion in that time. That's a lot of roof racks, which makes sense because they have a lot of uses. Folks rely on them to carry all kinds of things, from bikes and kayaks to suitcases and outdoors gear like Porsche's $7,000 roof tent for classy camping.
But it also reflects a lot of extra gas consumption, because adding roof-mounted cargo accessories to your vehicle has a definite negative impact on fuel economy. Depending on where you look and what vehicle you drive, you'll find estimated fuel-efficiency losses ranging from 2 percent for a small SUV carrying roof rails and crossbars only (without a carrier mounted), to nearly 20 percent for a sedan with a roof rack and carrier combo.
Let's talk about some terminology, too. For our purposes, roof rails are the bars that go front to back on the roof, crossbars are the separate components that go from side to side, and a roof rack is considered the whole setup, usually with another separate carrier piece to hold the cargo. Generally speaking, the more stuff on your roof, the more it impacts your fuel economy.
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It comes down to two primary factors, weight and aerodynamics. The first, at least with an empty roof rack, can have a relatively small effect. The exact weight of a roof rack is going to vary based on its size and material, of course, but as an example, a pair of Thule crossbars with one of the company's cargo baskets on top can tip the scales at under 40 pounds. It's not that much, but every 100 pounds of weight you add to your car can lower its fuel economy by up to 2 percent.
A bigger issue is aerodynamics — or lack of them. It turns out that pushing a car through the air is no easy task, and it gets more difficult as you go faster. The result is that, when you're cruising on the expressway, about 60 percent of the power sent to your wheels is being used up to battle air resistance. That's what happens when some rides today have worse aerodynamics than a loaf of bread.
Now, you measure how slippery a vehicle is with a complicated formula that provides a coefficient of drag, with the lower the number the better. And what's important for our purposes is that increasing the coefficient of drag by .01 — no units are involved because math — can increase your fuel consumption by .2 mpg.
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The good news is that there are a few ways to help mitigate how much fuel you're using when you've got a roof rack — starting with removing it when it's not in use. If the only time you load up your roof is for the occasional weekend adventure, there's really no good reason for keeping it attached.
When that's not an option, though, you can choose a roof rack built from lightweight aluminum instead of steel. Remember, a pound of the former can be as strong as two pounds of the latter. Just pay close attention to the overall weight of the roof rack because that's what's important here. Many of today's roof carriers are also engineered with aerodynamics in mind, and adding wind deflectors can make your passage through the air even easier.
With that in mind, it's worth pointing out that a hitch-mounted cargo carrier, which hides from the wind behind your vehicle, can be a much more efficient way to pull stuff. For instance, when Motor Trend put a Toyota Grand Highlander to the test, it had a 4-mpg advantage with the hitch-mounted carrier compared to the roof box.
But the best way to save gas with a roof rack — or without — is probably the least fun: Your car can be more efficient when you drive slower. According to the EPA, aggressive driving, defined as including speeding and rapid acceleration, can reduce fuel efficiency by 10 to 40 percent.
