The biggest improvement to FE in big rigs is a 60 MPH governor. I just did a cross-country drive from California to Tennessee and back, and I can't tell you how many trucks passed me doing 80+ MPH.
In fact, I never gave a second thought to California's 55 MPH limit for trucks until I tried driving 65 MPH in Arkansas. Those trucks pass you at a +15 MPH delta, even in heavy rain, and it's downright unsafe. As far as money goes, didn't somebody on this site do the calculations and determine that they actually make more money at 55 MPH than 70 MPH just because of the fuel savings?
BTW, MVT has been at it again:
Innovative Fleet Installing 3,500 Sets of Trailer Aero Improvers - Truckinginfo.com
Most of them are now sporting the Trailer Tail. Of course, they were driving 70+ MPH, too.