It increaed my cruising speed and fuel mileage from 8 to 11 mpg towing. I have put a little over 25k mi on the 5th since Sept of 2k, Have a little over 50k on the truck. Before the wing ( about 5k mi towing ) I was able to maintain 65 to 70 now I have no problem with 75 to 85. 40 - 60 mph cross winds are not a problem now as I can still maintain 75 without trailer swaying.
I get 15 mpg w/o a trailer and the wing on. 18 to 19 w/o the wing and no trailer. With out the trailer there is a wall at about 70 w/wing. Showed up big time on the track at MM last year.
Barry, make some different length struts and try different wing heights. I made several from 16" to 6" in 1" increments. at each fuel stop I would check the bug line and raise or lower the wing untill the bugline was just over the top of the trailer. The 6" is the lowest the my wing will go for bob tailing or my car hauler. Mine came w/ a 12" strut that put the bug line on my 5th about 18" above the cab. 14" strut raised the bug line to the top edge of the trailer. 16" was above the areodynamic eff. of the air foil and was worse than no wing.
Put a good coat of wax or silicon on the wing as the bug guts will bake on and petrify to the point that the kitchen oven will be easier to clean than the solar oven on the roof.
Just my experience and observations.
Jack