Of course, each year requires sizable improvements by automakers, and it’s too early to say whether they will have enough high-tech firepower a decade from now to reach 54.5 mpg.

But in the meantime, Olechiw says a number of tools are being applied that had not been considered by the EPA, such as cylinder deactivation on a 4-cyl. Volkswagen engine available in Europe. And diesels are proliferating more quickly than expected.

He says continuously variable transmissions, available for many years in hybrids, “did not play a big role” in the EPA technology roadmap, but now Honda, Subaru and Nissan have improved them and are using them in more mainstream vehicles.

As a result, the EPA expects additional CVT applications in the future.

Also on the transmission front, shift strategies factor prominently into the midterm assessment.

“There’s a lot of literature from the manufacturers saying how critical it is to make sure the transmission and the transmission calibrations are properly mated to the engine to get the full benefit of engine performance,” Olechiw says.

In 2012, the agency anticipated eight speeds would be the maximum number of forward gears, but Chrysler already is in production with a 9-speed.

Ford is expected to add a 9-speed automatic for its SUVs in ’15 and a 10-speed for its F-150 in ’16, according to a WardsAuto forecast. General Motors plans to begin production of a 10-speed automatic at its plant in Romulus, MI.

“For the midterm evaluation, we will include cost and effectiveness for these types of technologies,” Olechiw says.

Other powertrain strategies the EPA is benchmarking include high compression ratios (such as on Mazda’s Skyactiv gasoline engines) and dual-clutch transmissions, as well as the recent proliferation of downsized turbocharged engines.

All architectures are being explored, including conventional internal-combustion engines, mild hybrids, full hybrids, plug-in hybrids and range extenders such as the Chevrolet Volt.

“We’re also benchmarking electrified components such as motors and batteries,” he says.

Gasoline direct injection, which boosts both efficiency and power, also is drawing much attention from the EPA.

“We anticipated this being a key technology in both the 2012 and 2017 rules,” Olechiw says. “We’re seeing a fairly high penetration of GDI, and we expect that to continue.”