I picked up.5 mpg with deflector. It also made the truck not work as hard (I can watch the throttle opening and load on the motor with my dash command app) it goes down when the deflector is on.
And yes it needs to be mounted as far back as possible, the closer to the trailer the better. Even without my app I can tell the deflector works by the almost nonexistent amount of bugs on the front of the trailer. One of the best things I’ve done for towing!
I paid $75 for it and figured it paid for itself on our first 2k miles trip.