Way back in 2012, the US government released a relatively ambitious plan to increase US passenger fleet average fuel efficiency to 54.5mpg. Back then, we looked at some of the new technologies that automakers were adopting in order to meet this goal, plenty of which can now be found in our cars. But despite lots of hard work by the boffins in automotive research centers in the US and elsewhere, the 54.5mpg Corporate Average Fuel Efficiency (CAFE) goal is dead in the water.

Americans, it seems, are just too in love with their light trucks and SUVs to make it happen. That's according to a new report from the Environmental Protection Agency, the National Highway Traffic Safety Administration, and the California Air Resources Board. The three agencies have published a Draft Technical Assessment Report, "Midterm Evaluation of Light-duty Vehicle GHG Emissions Standards for Model Years 2022-2025" (PDF), that lays out the case for why we could meet the 2012 plan—which would have doubled fleet fuel economy, halved greenhouse gas emissions, and saved 12 billion barrels of oil and prevented 6 billion tonnes of CO2 from entering the atmosphere between now and 2025—but won't.

The report—which stretches out to over 1200 pages—spends plenty of time discussing cool technological advances, including improvements to gasoline internal combustion engines, better transmissions, mild (48v) and high-voltage hybrids, battery electric vehicles, fuel cell EVs, and more, but the bad news gets going in Chapter 12. The report projects that 46.3mpg is where we'll be when it comes to CAFE in 2025, a drop of 15 percent compared to where we'd hoped to be.

It's worth noting that the 2022-2025 CAFE targets were not finalized back in 2012 (as well as the fact that CAFE mpg numbers are not the same as the EPA fuel efficiency figures you or I might use when deciding what car to buy). But the assumptions that underpinned that target were based on a fleet that was two-thirds passenger cars and a third light trucks and SUVs. Now, the agencies have revised that based on consumer demand to a near-50:50 mix (52 percent cars, 48 percent trucks to be exact).

Despite the unwelcome findings in the report, there is quite a lot in there that is interesting. For one thing, we wouldn't even need to buy that many battery EVs, strong EVs (think Toyota Prius), or plug-in hybrid EVs to hit the target. In fact, EPA projects just three percent strong hybrids, two percent BEVs, and two percent PHEVs across the entire fleet by then.

Advanced gasoline engines would make up the bulk of our passenger fleet, with a mix of turbocharged engines (33 percent) and Atkinson cycle engines (44 percent). Expect plenty of engines to adopt cylinder deactivation, variable valve timing, and exhaust gas recirculation too. Nearly one in five cars will be a 48v mild hybrid, according to EPA.

The draft report is now open to a 60-day public comment period, with a final determination on just what the CAFE regulations will require for 2022-2025 due by April 1, 2018.