All other factors being equal, wind deflectors improve gas mileage on tractor trailer trucks by about 7.5 percent at highway speeds. Wind deflectors on top of the tractor, between the tractor and trailer, beneath the trailer and at the back of the trailer combine to reduce drag by 20 percent.
There are numerous wind-deflecting devices for the tractor, including a wind deflector atop the cab, which can include a sleeper bunk. Air fins on the side of the cab and skirts or deflectors running from the steering wheels to the drive wheels also help improve gas mileage. Trailer skirts, deflectors in front of the trailer tires and rear fins all help decrease the air resistance of the trailer, improving gas mileage.