by 
The Curious Bag
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According to a U.S. Department of Energy website, adding a car top carrier could reduce a vehicle's gas mileage by about 5 percent.
Function
A car top carrier affects gas mileage by adding wind resistance and creating more drag, therefore altering the aerodynamics of a car and lowering its fuel efficiency.
Significance
More than half of a car's engine power is devoted to overcoming aerodynamic drag when driving. A car top carrier increases aerodynamic drag, forcing the engine to work harder to maintain speed.
Considerations
A 2009 Consumer Reports study found that the addition of a popular car top carrier reduced a Toyota Camry's fuel efficiency from 35 mpg to 29 mpg at highway speeds.
Alternatives
There is a wide variety of back carriers and hitch-mounted carriers that can safely stow cargo without negatively affecting a car's aerodynamics and its gas mileage.
Expert Insight
The addition of a fairing may reduce noise from a carrier, but fairings are not known to have much impact in reducing a car top carrier's affect on gas mileage. Even empty racks have shown a negative impact on a car's fuel efficiency.
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