Narrow Mud Flaps
A variety of mud flap alternatives have been on the market for many years offering improved aerodynamic performance and fuel savings. The consensus is that these devices should be beneficial but the amount of fuel saved is hard to determine in individual tests.
One critical aspect of mud flap aerodynamics is specifying the correct width of mud flap for the tires. There are differences between wide-base singles and duals so that one size mud flap does not fit all. A mud flap that is too exposed to the airflow will increase aerodynamic drag and downstream issues.