The deflectors on the "big" trucks are as tall and wide as the trailer, and fairly close to it. Pick-up models are a novelty at best. The best fuel saving device is to slow down.
Not only are the deflectors close, big trucks also use cab extenders on the sides of the cab to close the gap between tractor and trailer.
I agree with Supreme that most of the air deflectors on PUs do little or nothing to help with air flow over the trailer as they arn't close enough to the trailer to work.
Mom , Dad , Scooter & K.C ( killer cat )
'96 F350 4dr. Dually, 7.3 5sp.
'92 Holiday Rambler 34'
REAL TRUCKS DON'T HAVE SPARKPLUGS !
It is interesting that when this subject comes up there are both somewhat equal pro/con comments. Either you like or you don't.
It made a difference with my prior gas powered TV (84 Chevy Dually) both with head winds and on the freeway albeit I averaged 7 mpg any improvement is good. There were times (HWY 152 West @ Los Banos Ca) where the headwind was strong enough to hold my speed down to a max of 40. I purchased the wind deflector and was no longer held back.
With that said the Duramax is not short on power and the difference is now slight. I have not traveled with it only 3 different times to South Lake Tahoe (HWY 50) all of which the truck ran hot, with the deflector (same conditions) no issues. I also see a benefit with head winds and there are less bugs on the front of the trailer.
I bought a Taylor Wing which I do not know if they are still being sold. The last time I checked the price doubled from my original purchase, I would not purchase again based on price.
2015 303 RLS GD Reflection, 2005 Chevrolet Silverado 2500 Duramax