DETROIT — When General Motors CEO Dan Akerson vowed earlier this month to cut the weight of new models 15 percent by the 2016 model year, he put his engineers on a rigorous schedule.

Is it doable? Yes, but it's a stretch, says one knowledgeable industry consultant.

"It will be a big effort," said Richard Schultz, managing director of Ducker Worldwide in suburban Detroit, an adviser on mass reduction to GM and other automakers. "That's a lot of work, and GM doesn't have a lot of time."

For instance, GM would have to shed about 600 pounds from a V-6-powered large sedan such as the Buick LaCrosse, which weighs 4,045 pounds.

Engineers could save as much as 200 pounds by substituting a four-cylinder engine for the V-6. The rest would come from lightweight materials.

Schultz said GM will avoid large amounts of carbon fiber reinforced plastic, and it is unlikely to introduce an aluminum-bodied car, as Audi and Jaguar have done.

Instead, GM will use:

• More high-strength steel for the body-in-white.
• Magnesium for selected parts such as transmission cases.
• More aluminum for doors, decklids, hoods and structural parts.

Like other CEOs, Akerson doubtless feels a sense of urgency to meet federal corporate average fuel economy standards, which rise to 35.5 mpg by the 2016 model year.

In his Houston speech last week, Akerson hinted at GM's intention to use more aluminum and high-strength steel.

GM has begun using spot welding to attach aluminum to the body-in-white, saving money and weight by eliminating rivets, which can add as much as two pounds to a car's weight.

The new spot welding system disrupts the oxide on the surface of an aluminum component, ensuring a stronger weld than before. That, in turn, eliminates the need for rivets, GM says.

GM offered a glimpse of the technology's potential with the 2014 Chevrolet Corvette Stingray, company spokesman Dan Flores said.

Spot welding "allows engineers to look for more innovative ways to use aluminum beyond traditional uses" such as hoods, decklids and doors, Flores said.

Translation: Look for GM to use a lot more aluminum in structural components in the body-in-white.

GM also hopes to save money by introducing a high-strength steel that is under development. Last year, GM said it had invested in NanoSteel Co. of Providence, R.I., which has developed a nano-structured steel alloy.

Automakers like high-strength steel because they can make parts thinner, saving weight. But typically, steel alloys must be hot-stamped, which raises manufacturing costs.

NanoSteel is developing a high-strength steel that would be cold-stamped, at less cost.

"If it's cheaper, you can use more of it," Flores said. "It's still in trial, but that's where we think the competitive advantage is."

While GM is investing in aluminum and high-strength steel, one might argue that the company is still hedging its bets.

Take the 2014 Chevrolet Silverado pickup, which goes on sale this spring. GM says it cut 59 pounds by using an aluminum hood, front control arms and steering knuckles. It also used high-strength steel for two-thirds of the cab structure.

But GM could have saved more weight by introducing an aluminum cargo box, cab or bumpers. Mark Reuss, president of GM North America, has noted that fuel-conscious buyers can opt for the Chevrolet Colorado compact pickup.

But Schultz speculates that GM will work harder to cut the weight of its next-generation Silverado. "Why spend the extra money until you have to?" Schultz asks.

What about more exotic materials such as carbon fiber reinforced plastic? The Corvette features a carbon fiber hood, but this material is still too expensive for mass-market vehicles.

To cut cost, GM formed a partnership in 2011 with Teijin Ltd., a Japanese producer of carbon fiber. The company has opened a technical center in suburban Detroit to develop a cheaper way to manufacture carbon fiber reinforced thermoplastic.

Carbon fiber weighs one-fourth as much as conventional steel and is 10 times stronger. But it takes a long time for the resin to set in the mold.

Teijin is trying to reduce the cycle time required to produce carbon fiber parts to less than a minute. But that technology isn't ready, Flores said.

Because carbon fiber is not ready for prime time, that leaves aluminum and high-strength steel as the materials of choice -- especially for a vehicle's front end.

Look for engineers to use more aluminum hoods, bumpers, suspension components and possibly engine cradles.

But GM "is not going to use any technology that they are not already comfortable with," Schultz said. "Everything is happening just a little bit faster, but it's something they know how to do."