More use of weight-saving materials such as aluminum, carbon fiber and high-strength steel are boosting gas-mileage in new cars, but also are complicating and adding cost to repairing them after accidents.

Automakers are under intense pressure both to save weight for gas mileage and to meet stricter crash-test standards. But the sometimes exotic materials needed to meet those goals mean replacement parts not only cost more, but often require elaborate repair techniques that are taxing skills in some body shops.

They also raise concern about dangerous repair shortcuts or substitution of cheap steel parts instead of costly ones that came with the car.

“The consumer needs to understand the new vehicle they bought has specific requirements that need to be met to be as safe when they get it back from repair as it was before the accident,” says Doug Irish, president of VeriFacts Automotive, which monitors and advises body shops.

• It’s getting more expensive to fix cars made of new lightweight materials
• Not only to do parts cost more, but repairs involve special equipment and techniques
• Some repair shops benefit, others don’t

As automakers strive to meet federal mandates, more have turned to lighter materials, including:

•Aluminium. Automakers will increase aluminium in cars from an average of 327 pounds in 2009 to 550 pounds by 2025, the Aluminum Transportation Group says. The weight savings are dramatic: Replacing steel with aluminium in doors of the 2014 Cadillac CTS sedan, for instance, saved 55 pounds.

•Carbon fiber. As the cost of carbon fiber drops, the composite is showing up in more parts. While already a preferred material in high-end sports cars, it’s moving down-market. BMW is using a carbon-fiber body for its i3 small electric car.

•High-strength steel. Lighter for the same structural strength, it’s being used for more body parts to cut weight and still meet roof crush and front-impact rules.

But beyond the cost of the new crash parts, each material requires special methods and tools for the repairs. Special equipment — from high-strength drill bits to new frame racks — also is adding to repair costs.

All this also is creating haves and have-nots among body shop businesses based on whether they can afford costly new equipment.

“It’s something that’s going to differentiate our industry,” says Darrell Amberson, a vice president for LaMettry’s Collision with seven shops in greater Minneapolis. It just bought an $80,000 rack for repairing auto chassis.

And some of the materials can’t be repaired, only replaced. Workers often can’t just weld in new steel or straighten out damaged sections when high-strength steel are involved. The heat of welding can weaken the part.

“Where before you could cut a small piece in,” says Bob Keith, training director for Carstar, a network of more than 400 body shops, today “you might have to replace the whole thing.”