Tough new fuel efficiency standards have pushed automakers to look at aluminum, carbon fiber and other lightweight metals so they can make lighter vehicles that release fewer emissions into the atmosphere.

BMW is making its i3 model with a carbon fiber composite frame, and Audi and Jaguar have replaced steel bodies with aluminum in some models. The automakers are racing to meet new rules, which for instance require cars to reach an average mileage of 54.5 mph in the United States by 2025.

Steelmakers have been pushing back against the growing use of alternate metals in one of the biggest markets for steel. They have developed lighter grades and new designs for auto parts that weigh less but are still made of steel. They also are pushing for federal regulations that look at not only the emissions that come from a car's tailpipe, but also the factory that built the metal that went into the car and how much of the metal is recycled after the car ends up in the junkyard.

A car's entire life cycle must be considered to account for the full environmental impact, said Lawrence Kavanagh, president of the Steel Market Development Institute, a joint effort by major steelmakers to promote the use of steel.

Here's why: An aluminum-framed car would emit less than a steel-framed vehicle if it weighed less. But aluminum production is far more energy-intensive than steelmaking, and can release as much as five times as much greenhouse gases, Kavanagh said. The production of carbon fiber and magnesium – two metals that also are making inroads in cars – also results in more emissions.

Any reduction in emissions over the 200,000-mile life of the vehicle would fail to make up for all the extra gases released into the atmosphere during the production phase, Kavanagh said.

The Aluminum Association, a rival trade group, contends the lighter metal is fact better for the earth. A study it commissioned found every pound of aluminum used in an automobile saves 20 pounds of CO2 emissions over the lifetime of the vehicle.

Kavanagh said the contention is short-sighted and looks only at the emissions that occur when the vehicle is being driven. Studies by the University of California-Davis and the University of California-Santa Barbara have found that the emissions from materials and vehicle manufacturing can account for half a vehicle's total emissions.

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Steel has environmental advantages, particularly when viewed over a vehicle's entire life cycle instead of just through the measurement of tailpipe emissions, new U.S. Steel Chief Executive Officer Mario Longhi told a business crowd at a recent speech in Pittsburgh, according to a transcript provided by the company.

"If vehicle lightweighting is accomplished by substituting materials that are emissions-intensive to make, then the net impact on the environment is more greenhouse gases," Longhi said. "The steelmaking process emits one-twentieth to one-fifth the amount of greenhouse gases of other materials, and steel is by far the most recycled material on the planet."

Steelmakers have been lobbying the U.S. Environmental Protection Agency to consider new emissions regulations that could for instance extend credits to manufacturing processes that release fewer greenhouse gases.

What's at stake is whether the steel industry can defend its market share in the automotive industry, which accounts for a fourth of its overall business. Much of the steel made at mills in Northwest Indiana gets loaded onto Detroit-bound semi-trailer trucks and ends up in cars, trucks and sport-utility vehicles.

Mill operators ArcelorMittal and U.S. Steel have been developing advanced high-strength steels that are lighter than conventional steel. Steelmakers also have been working on new designs for steel parts that will lessen the weight of vehicles.

Those efforts have been paying off. A major domestic automaker recently started using a new design for a front lower control arm, which it had made with aluminum but now makes with steel, Kavanagh said.

"This is about defending existing market share, and also growing market share," he said.