Motorists can get better fuel efficiency with their windows closed and air conditioning on when at highway speeds, according to the Automotive Aftermarket Suppliers Association (AASA). During stop-and-go traffic, however, it's more fuel efficient to open the windows.With the windows closed on the highway, the vehicle's aerodynamics are improved as there is less drag on the vehicle, according to Frank Hampshire, AASA director of market research. "In city driving, you will see a slight fuel economy advantage by opening your windows, but the use of air conditioning at higher speeds can produce a fuel savings," Hampshire said.Some consumers with malfunctioning car air conditioners may opt to roll the windows down rather than make repairs, but that's a false economy, Hampshire noted. "The fuel efficiency savings may not be enough to pay for the repair of a non-functioning air conditioner. However, for motorists with malfunctioning systems who have decided to endure the heat, fuel savings make air conditioner system repair worth considering," he said. "Furthermore, a functioning air conditioner system might be necessary for proper functioning of the windshield defroster in late model vehicles."