A lawsuit claims General Motors Co. treated the police better than it did average citizens when taking care of a defective part in 2007 and 2008 Chevrolet Impalas.

The lawsuit alleges that Impalas from the two model years have defective spindle rods, which connect the suspension to the rear wheels. The defect causes the wheels to misalign, which makes the tires wear out faster. The tires could also wear out unevenly, increasing the risk of a blowout.

GM fixed the part on police versions of the Impala three years ago but didn't correct the same problem in hundreds of thousands of other Impalas, according to the lawsuit filed last week in Detroit.

Donna Trusky of Blakely, Pa., who bought a new Impala with Goodyear tires in February 2008, claims that before she reached 6,000 miles, the tread on her rear tires was so worn she had to replace them. Typically, tires should last for 30,000 miles or more. Her lawyers are asking the judge to certify her lawsuit as a class action.

According to the lawsuit, GM sent a bulletin to dealers in June 2008 telling them to replace the spindle rods and tires on affected police vehicles. It also authorized dealers to reimburse police who had purchased replacement tires as long as the reimbursement request was made before July 31, 2009.

But GM allegedly didn't offer the same remedy to non-police owners. The company sold a total of 423,000 Impalas from those model years. GM spokesman Alan Adler said 23,800 of those cars became police models.

Adler confirmed that GM issued a service bulletin for police cars from the 2007 and 2008 model years because of rear suspension problems. But he said the company wouldn't comment further.

"We routinely do not comment on matters in litigation," he said.

Trusky's Chevrolet dealer paid to replace the tires and have the front end of her car realigned, but didn't replace the spindle rods or mention the police bulletin. Trusky continued driving the car, but by the time she hit 24,000 miles two years later, her rear tires were worn out again. This time, she paid $289.77 to replace them. She says that amount should have been covered under her warranty.

At least 30 other drivers have sent complaints about the issue to the National Highway Traffic Safety Administration. While none has reported an accident or injury related to the problem, several drivers said the rear of the car tends to swing out, especially when driving on snow or ice. NHTSA hasn't opened an investigation into the Impala's suspension system, which is usually the first step in the process that can lead to a safety recall.