When you’re adding roof rack cross bars to your car, you’re thinking about utility, not fuel consumption. Whether it’s for hauling kayaks, bikes, or extra cargo, these bars give you more storage space. But, there’s a hidden cost — they can hurt your gas mileage. Is the convenience worth the price? Let’s find out.
Understanding the Impact of Aerodynamics
The main reason roof rack cross bars reduce fuel efficiency is simple. They affect your car’s aerodynamics. When you drive, your vehicle is designed to minimize air resistance, but when you put anything on top, especially a roof rack, you mess with that.
Adding a roof rack can decrease fuel efficiency by up to 25% on highway speeds, especially if the rack isn’t designed to be aerodynamic. The more wind resistance you add, the harder your engine works, which burns more fuel.
Taiebat et al. noted that shape alteration could be treated using roof racks (Taiebat et al., 2018). Chen et al. estimated that a roof rack could increase a passenger car’s fuel consumption rate by 0–25% (Chen and Meier, 2016). Thomas et al. reported that using a roof rack at 80 mph could reduce the fuel economy by almost 27% (Thomas et al., 2014)
https://www.sciencedirect.com/science/article/abs/pii/S0301421516300714
Even empty roof rack cross bars can have an effect. CarAndDriver.com found that a roof rack alone could reduce mileage by about 12%. It’s less significant than a fully loaded rack, but it still matters.
In our 75-mph highway fuel-economy test, we found the fuel economy of our long-term Kia Carnival improved by 3 mpg (or 12 percent) with its factory crossbars removed.
https://www.caranddriver.com/news/a42596866/tested-roof-rails-affect-gas-mileage/
The Effect of Weight on Fuel Efficiency
Weight is another factor, but it’s less important than aerodynamics. When your cross bars are loaded up, the extra weight does cause your car to burn more fuel. The Environmental Protection Agency (EPA) suggests that for every 100 pounds added to your vehicle, you lose about 1-2% of your fuel efficiency. However, this is less of a problem with roof racks, as the air drag plays a larger role compared to weight.
An extra 100 pounds in your vehicle could reduce your MPG by up to 2 percent. The reduction is based on the percentage of extra weight relative to the vehicle’s weight and affects smaller vehicles more than larger ones.
https://afdc.energy.gov/files/u/publication/10_G00888_savings_WEB.pdf
That said, if your cross bars are loaded with heavy items like luggage or cargo boxes, you’ll notice a further drop in gas mileage. The combined effect of the weight and reduced aerodynamics will have a noticeable impact.
Different Vehicles, Different Effects
Some cars are more affected by roof racks than others. For example, SUVs and pickup trucks already have a boxier shape, which means adding a roof rack won’t change their aerodynamics much compared to sleeker cars like sedans or sports cars. If you drive a smaller, more aerodynamic car, expect a bigger hit on your fuel economy.
A study by the National Renewable Energy Laboratory showed that for smaller vehicles, like compact cars, roof racks can reduce fuel efficiency by as much as 15-25% on highways. Larger vehicles showed smaller reductions, around 10%, because their shape already causes significant drag. This information aligns with real-world testing data, showing different impacts on various vehicle types.
Even without the kayak, the roof rack itself makes your car less efficient. One study, from Berkeley Labs and the National Renewable Energy Laboratory in 2015, using crowd sourced data, reported that the loss from a roof rack alone can be up to 25%.
https://www.daytondailynews.com/local/roof-rack-makes-car-less-fuel-efficient/SCEX7XYAYNEETEFXIZL7LS5K5M/
Fuel Savings Tips with Roof Racks
Removing roof rack cross bars when they’re not in use is the simplest way to prevent any reduction in gas mileage. The DOE recommends taking them off when they’re not needed, as even bare racks can lower fuel economy by 1-2%. This may not seem like much, but it adds up over time, especially for people who do a lot of highway driving.
Reduce drag by placing items inside the car or trunk rather than on roof racks, which can decrease your fuel economy by up to 8% in city driving and up to 25% at Interstate speeds.
https://www.energy.gov/energysaver/fuel-economy
Additionally, choosing aerodynamic cross bars or racks designed to reduce drag can help lessen the impact. Some newer racks are built to be more fuel-efficient. For example, racks that sit lower on the roof, or have a more streamlined shape, may only cause a small reduction in gas mileage compared to bulkier, older racks.
Real-World Testing
Testing done by Consumer Reports showed that a compact SUV lost nearly 11% of its fuel economy when carrying bikes on a roof rack. In contrast, when the same load was placed on a rear hitch mount, the fuel consumption dropped only by 5%. These real-world experiments show that placement matters just as much as weight.
Our tests revealed that the sedan’s fuel economy took a significant hit with the rooftop carrier. The Altima got nearly 48 mpg when traveling at 65 miles per hour with nothing on the roof. When just the roof rack was added, the car’s mileage dropped by almost 11 percent, or 5 mpg. But with the rack and rooftop carrier installed, the overall drop was 19 percent, a loss of 9 mpg.
https://www.consumerreports.org/fuel-economy-efficiency/how-rooftop-carriers-affect-fuel-economy/
The Cost of Convenience
It’s clear that roof rack cross bars do have a measurable impact on gas mileage, but it’s a trade-off. If you need to haul gear, you can’t avoid using a roof rack. The key is to minimize the damage by reducing drag where possible.
Always consider alternatives, such as rear-mounted racks or even trailer hitches. These often have less impact on fuel efficiency, although they come with their own downsides.
Final Thoughts
In short, the convenience of roof racks comes with a cost to your fuel efficiency. Whether you’re using them to carry bikes, skis, or just extra luggage, be prepared for a hit to your gas mileage. Studies show that cross bars can cause anywhere from a 5% to a 25% reduction, depending on your vehicle and what you’re carrying. The best way to avoid this is to remove them when not in use and opt for aerodynamic designs when possible.
For those looking for a balance between utility and efficiency, it’s important to be mindful of how and when you use your roof rack. Keeping your setup as streamlined and light as possible is your best bet for minimizing fuel costs while still getting the most out of your vehicle’s storage capabilities.
Always verify with credible sources like the EPA, DOE, and real-world testing before making decisions on your setup.
