Not much, if you're talking about the wing-looking things intended to divert the air up-and-over the 5er. Nobody has done a conclusive analysis, opinions vary from worthless to maybe a little bit (from 11.2 to 11.3).

Another one I've seen are the vortex generators you can put on the rear sides of the trailer. They're intended to break up the air pattern at the rear of the trailer & gain mileage that way. I think the jury is still out on them.

Regards, DBF

 

For what our 1st Gen truck lack in aerodynamics, we make up for with our good looks..

 

Any air deflector needs to be within 2 feet to be effective, and closer is better. Any further out and it just screws up the airflow worse.