Installing any large rectangular or bulbous object onto a car or truck will cause more air resistance which reduces your vehicle’s fuel efficiency. You will notice a difference in your vehicle’s gas mileage when hauling a roof top tent, or even a bare roof rack. Studies show that depending on the weight, material and shape of the roof rack and roof top tent, hauling the entire setup could cause up to a 35% decrease in fuel economy on the freeway, but the average is a 17% decrease.