These devices close the gap between the tractor and the trailer by extending the vehicle side surface into the gap while ideally not interfering with trailer swing and dip motions.
The purpose of cab and roof extenders is to restrict the airflow from entering into the trailer gap by shortening the effective tractor/trailer gap width to route air over and around the gap to the trailer.
Without extenders, air leaving the rear edge of the tractor would hit the front edge and face of the trailer causing an increase in drag
Due to lower drag
Cab extenders are vulnerable to damage in very tight and sloped circumstances
“We continue to update our fleet with more fuel efficient post-2014 U.S. Environmental Protection Agency emission compliant engines, install aerodynamic devices on our tractors, and equip our trailers with trailer blades, which all lead to meaningful improvement in fuel efficiency.” — Knight Transportation
“Appearance and detail of our equipment means a lot to our drivers. Drive wheel fairings don’t just save us money, but our drivers like them too.” — Brad Pinchuk, Hirschbach
“Sleeper tractor aerodynamics have been finely tuned by all OEMs. Eliminating various features can add 10% to a fleet’s fuel expense,” — Chief engineer at a major truck builder.
The Confidence Matrix has been issued to help fleets make decisions about tractor aerodynamic devices.