To get the engines ready, Cummins made changes that would reduce parasitic losses.

“Around the water pump, fuel pump and oil ring, we made the hardware modifications that would improve the overall efficiency of the engine,” Lavengood said. “We expect about a 2% improvement in fuel efficiency.”

Volvo, meanwhile, said it has received certification for its VNL, VNM, VHD and Volvo Autohauler (VAH) models.

“Volvo is committed to leadership in fuel efficiency and to reducing the carbon footprint of our operations and products,” said Göran Nyberg, president of Volvo Trucks North American Sales and Marketing.

To achieve certification, Volvo Trucks said, it improved vehicle aerodynamics, including tweaking mirror heads, redesigned hood mirrors and more features below the bumper and side fairings.

“They’re giving us the leeway to be very good in some areas,” Frank Bio, Volvo’s product manager of trucks, said in a telephone interview from the company’s Greensboro, N.C., base. “The intent is to improve the overall trucking fleet rather than use dirty engines or things not aerodynamic.”

Likewise, Mack said its full lineup of model year 2014 trucks and tractors had been certified by EPA and NHTSA.

The company said the primary changes under the hood involved engine tuning or fuel map settings. Other changes were made in tire selection — a greater focus on low rolling-resistance tires — and aerodynamic options.

Volvo Trucks is a division of Volvo Group, based in Gothenburg, Sweden, that also owns Mack Trucks Inc.