DETROIT — General Motors is about to start making parts with a technique that allows it to weld steel and aluminum together.

No other automaker can do it, and it’s costly and time consumingy when rivals convert a vehicle from traditional steel construction to aluminum.

If GM perfects the process, which it’s testing on parts for the new Cadillac CT6 luxury sedan, the automaker will be able to build vehicles that combine aluminum and steel parts faster and cheaper. The customer gets fuel savings and better performance. GM gets a reduction in the time and money it takes to get vehicles to market faster and at a lower cost.

The time and money it takes converting a factory from building a steel vehicle to one that combines steel and aluminum would be slashed while GM’s competitors spend more to do the same thing.

Ford had to gut its Rouge assembly plant in Dearborn, Mich., to convert the F-150 from a steel body to aluminum. It lost months of production and spent many millions of dollars ripping out the old assembly line and installing a new one. If ongoing tests on the hood and rear seat of the CT6 pay off, GM could make the same conversion in a matter of weeks when the Chevy Silverado pickup adopts an aluminum body a couple of years from now.

Among reasons automakers haven’t been able to weld steel to aluminum, the two metals melt at vastly different temperatures. At the temperature where aluminum melts, steel is only getting warm. The automaker has 19 patents on the process that can do it.

“Weight reduction is critical to improving fuel economy, which, despite lower gas prices, still is important for consumers,” Autotrader senior analyst Michelle Krebs said. “When gas prices rise, as they surely will, buyers of trimmer General Motors vehicles like the Chevrolet Malibu will appreciate the automaker’s light weighting efforts.”

Vehicle weight is a vicious cycle. If a car’s basic structure is heavier, the automaker has to use a bigger engine to get competitive performance. The bigger engine requires more supports to hold it in place, which make the vehicle heavier still. That weight requires bigger brakes to stop the vehicle, and those brakes make the car heavier yet.

The reverse is true. Every ounce engineers take out of one part of a car allows them to save more weight elsewhere.

After trailing the industry for years, GM has become the lightweight leader. Every new vehicle benefits from what GM engineers learned on the predecessor. The Cadillac ATS surprised everyone by being hundreds of pounds lighter than the BMW 3-series when it debuted in 2012. Every vehicle GM introduced since has shed hundreds of pounds from its predecessor, and most are the leaders in their class.

Some examples:

•The 2016 Chevrolet Cruze compact sedan is 250 pounds lighter than the 2015.

•The 2016 Chevrolet Malibu shed 300 pounds.

•2016 Chevy Camaro, 400 pounds.

•2016 Buick LaCrosse, 300 pounds.

•2017 Cadillac XT5 280, pounds lighter than the SRX it replaces.

Most impressive, the big Cadillac CT6 sedan is lighter than the BMW 5-series midsize, which is a full size class smaller than the Caddy.

Other weight-saving projects in the works at GM include magnesium parts. A diecast magnesium frame for a car door is so light that it feels like plastic when you pick it up, and replaces seven parts with one.

Weight reduction doesn’t just rely on lightweight materials. GM engineers are ruthlessly focused on efficient design.

“Structural efficiency is a relentless grind to reduce mass,” said Warren Parsons, GM chief architect for body structures.

Other examples include combining 25 parts under the CT6’s front fender into a single aluminum casting that’s lighter and takes less time to install in the car.

GM also cut pieces the size of fingernail clippings from the edges of steel stampings in the Malibu’s structure. The extra steel didn’t add anything to the structure, but each one increased weight by a few grams. All it took to remove the unneeded mass was a cleverly designed die to make the part, so the weight savings didn’t add anything to the car’s cost.

“There are huge savings when you reduce the cost and complexity of a car,” said IHS Automotive senior analyst Stephanie Brinley.