Cost is the crux of this debate.
"That's the biggest problem," Thomas Weber, head of Mercedes-Benz product development, said in an interview this year. "Customers will accept new technology, but in the end their budget is limited."
In a 2015 Power study on consumer interest in new tech, "fuel efficient technologies came out fairly low," said Renee Stephens, vice president of U.S. auto quality for Power. Stephens points to low gasoline prices as one factor.
Part of the problem is connecting green tech with tangible benefits. Buyers aren't interested in features such as solar glass roofs and improved aerodynamics because the consumers don't perceive the features' value.
Even the driver-friendly mpg indicator "has a very low level of interest. Forty-four percent of consumers didn't see the point," Stephens said.
If consumers don't care about green cars, goes the automakers' argument, it's foolish to build them.
To complicate matters, there are many ways to improve fuel economy, but no clear route to success. Even within major categories, such as multispeed transmissions, automakers have numerous options.
Some automakers are taking an all-of-the-above approach.
"It'll be important to prove fuel efficiency among all fronts," said Raj Nair, Ford product development chief, in an interview at the 2016 Detroit auto show.
Nair ticked off a laundry list of Ford's fuel-saving efforts, including EcoBoost engine improvements, lightweighting, fuel cells and a $4.5 billion investment in hybrids and EVs.
Ambitious investments in green tech trickle down to dealer lots as higher prices.
For example, the starting sticker price for a 2015 Honda Civic is $19,325, but for the 2015 Civic Hybrid it's $25,525. Prices include shipping. The annual fuel cost, according to fueleconomy.gov, is $800 for the gasoline Civic and $600 for the hybrid. Honda discontinued the Civic Hybrid after the 2015 model.
The case is similar for diesel powertrain options. The 2016 Ford F-250 XL with a gasoline engine starts at $33,580. The same truck with a diesel engine jumps to $42,060. Prices include shipping.
The EPA insists 54.5 mpg by the 2025 model year can be achieved affordably through improving the gasoline engine and implementing nonpowertrain strategies such as lightweighting and improving aerodynamics.
Estimating the cost of these changes is difficult. In 2015, a National Research Council expert panel used EPA, NHTSA and automaker data to calculate direct manufacturing costs of different tech combinations. (See graphic, Page 3.) The panel did not consider retail markups, so its calculations were below the expected sticker prices. Nor did the panel take into account automaker strategies to reach fuel economy targets, such as trading compliance credits or averaging vehicles' mpg across the fleet.
For a midsize car with a gasoline engine, the panel estimated new fuel-saving improvements could cost between $1,181 and $1,658 per car by 2025, in 2010 dollars and before credits are applied. How do automakers get there? Through a mix of engine friction improvements, an eight-speed automatic transmission, drag reduction, 10 percent weight loss, low-rolling-resistance tires and more.