DETROIT (June 11, 1:05 p.m. ET) -- The auto industry has signed on to proposed federal mandates to dramatically improve vehicle fuel economy. But for automakers to meet new standards, some technologies will have to be invented.

“The auto industry has agreed to meet targets that we don’t know how we’re going to meet,” says Tom Baloga, vice president of engineering at BMW of North America. “We’re ready to make commitments to tough goals. What we need is time and we need certainty.”

The Obama administration, the Environmental Protection Agency and the National Highway Traffic Safety Administration have widespread industry support for requiring nominal fleet averages of 54.5 mpg in 2026. (Because of various exceptions and credits, the real-world average likely will be in the low 40s.) Current rules require a 2012 model year industry average of 29.7 mpg.

“To reach those numbers, there is technology that is going to have to be invented,” Baloga says.

Already used extensively are turbochargers, multispeed transmissions and aerodynamic improvements. But new technologies are in the works, and automakers are betting on a few that seem plausible.

Some of the technologies with plastics connections include:

• Lighter windshields: Plastic or thinner glass would trim several pounds, but there are problems with both.

Plastic, standard in the motorcycle industry, is more expensive than glass for vehicle windows and tends to make the vehicle interior hotter. Thinner glass can lead to noisier cabins, says Jim Shepherd, who was just named president of Carlex Glass.

In recent years, glass suppliers such as Carlex have looked at changing the thickness of all windows to reduce total weight, he says. Heavier side windows can reduce the cabin noise that results from thinner back glass, Shepherd says, and the windshield can be made thinner by improving the acoustic vinyl between the windshield’s two glass layers.

•  Heat capture: Harnessing heat emitted by vehicles could slash fuel use. Last year, Ford and General Motors began experimenting separately with thermoelectric technology, which channels engine heat through semiconductors that convert it to energy. Ford’s program uses thermoelectric technology developed by Amerigon Inc., while GM is developing the technology internally.

A different technology, thermophotovoltaics, is under development at MTPV Corp., an Austin, Texas, technology supplier. The technology uses semiconductors and other devices to capture heat and convert it to alternating current to run vehicle electronics and cooling, potentially replacing vehicle alternators, says Vice Chairman Dave Mather.