The German company Spedition Boll tested a modified Mercedes Actros for 6 months and got a 14% reduction in fuel consumption compared to a normal truck and trailer. The Actros 1841 had all of its 'extras' stripped - spotlights, horns, railings, sun visors, etc., in other words, everything that normal truck drivers just love to add to their cabs. The front mirror was replaced with a camera and deflectors were mounted on the A-pillars. It got SuperSingle tires on the drive axle. This was mated to Krone's Ecoliner trailer, which have characteristic side and wheel skirts.
Four test trucks (two MB Actros 1841's and two MAN TGX 18.400's) with curtainsider trailers were designated to the same route as the modified one, to serve as a comparison. All four had to have their 'extras' stripped (which the drivers weren't happy about) so that the comparison would be fairer. After 6 months of testing, the four trucks averaged 29.2 l/100km (8 mpg), while the modified truck got 25.6 l/100km (9.2 mpg).
The main problem with the trailer's side skirts was how easily they can be damaged while docking, either by the dock's side guides, or when the ramp angles downward too steeply. When loading from the side, the forklift's driver must be very careful. Also, the SuperSingle tires tend to lose traction in snow.
Spedition Boll is very happy with the test's results. The price of converting more trucks hasn't been calculated, but each of the components will be tested individually. Experimenting with a rear diffuser is being considered.