Cab & Roof Extenders
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.
Benefits
Fuel Savings (If Coupled with Higher Capacity and Higher Voltage Electrical System)
Challenges
Not Available for Most Vehicles as of March 2017
Increased Complexity of Electrical System
Higher Electrical System Voltage and Power Output Required
Reliability Issues
Increased Maintenance Costs