Side skirts are still king when it comes to fuel-saving trailer specs and add-ons among fleets tracked in the North American Council for Freight Efficiency's annual study. Tom Berg has more in the Trailer Talk blog.

Almost nine out of 10 recently purchased box-type trailers are equipped with the low-hanging deflectors, according to NACFE's 2018 Annual Fleet Fuel Study. The council has been doing the annual studies since 2012.

That high purchase rate of skirts shown in the ’18 study is for the 20 fleets participating, which include many of the better-known and progressive operations in the country. There are several reasons for their adoption, including California’s mandate that began in 2010, says Mike Roeth, NACFE’s executive director.

To save fuel and reduce greenhouse gas emissions, any trailer 53 feet or longer running in California must have side skirts and low-rolling-resistance tires. So if there was a chance an out-of-state fleet’s trailers would venture into the state, that fleet began buying them.

“Since then, price came down, durability went up, and weight came down,” Roeth says. Those fleets "are now finding that as they’ve gotten up in scale, they’re really paying off.” Because of wide variances in operations, fuel savings range from 1% to 10%, though 3% to 4% is more typical.

“Of course, there are skirts and there are skirts… The closer they are to the ground, the more fuel saving there is.” Shallower “mini skirts” don’t save as much in fuel, but might be desirable to avoid damage from high driveway aprons and such.

Side skirts comprise the most widely adopted technology among fleet buyers of van and reefer trailers, NACFE’s ’18 study says. And the graph shows that the steep adoption curve started in 2007, three years before the California rules began phasing in. Tray- and boat-shaped fairings mounted ahead of trailer tandems have also shown value, though their acceptance by fleets is limited.

Other aerodynamic devices seeing serious adoption rates on trailers are vented mud flaps, at about 47% (probably because flaps are required anyway, and types that reduce drag by allowing air to pass through don’t cost much more); and tail fairings, at about 32%.

“Fleets bought into trailer tails but then pulled back because of trouble getting drivers to close them,” Roeth commented. Interest has rebounded in the last few years.

Trailer nose fairings, or "nose cones," and vortex generators (placed on side walls near the rear of trailers) have limited followings, and sales have plateaued since 2009 or so, the study says.

Other fuel-saving trailer equipment that has seen higher adoption by fleets includes automatic tire inflation systems and lightweight components. Many fleets use tire inflators “because the air pressure doesn’t get checked” by drivers, and trailer manufacturers have cut weight through design advances, like thin walls.

Wide-base single tires on trailers only went so far in popularity. Fleet managers found that flat tires weren’t being noticed by drivers, and decided to go back to dual wheels and tires, Roeth says.

The latest Annual Fleet Fuel Study covered many subjects, primarily concerning road tractors. It found that in 2017, fleets averaged 7.28 mpg, up somewhat from ’16. NACFE’s studies and information are free. Download copies of the 2018 and earlier studies, as well as Confidence Reports about various fuel- and money-saving technologies, at NACFE.org.