If you are trying to save on gas. You need to get the roof rack off your car.
More than 50 percent of engine power goes to overcoming aerodynamic drag on the highway. Even an empty bike or ski rack can shave about 6mpg off your gas milage.
Also check your air pressure on your tires at least once a month. Underinflated tires can cost you 1mpg. Underinflated tires wear out faster.