What's wrong with this old car?

While driving my wife’s car I noticed a knocking sound. I checked the front tires to make sure that the nuts were tight and they were. When I looked at the rear tires I noticed that both of them were very worn on the inner side of both tires. They were worn down to the steel belt. The tires are fairly new. The outer part of the tires were not worn. Her vehicle is a 98 buick century. Please advise.

There is something amiss in the rear suspension, resulting in bad camber. PLEASE do NOT drive the car on these tires!

As a first step, I would suggest that you have the car towed to a reputable tire dealer for immediate replacement of those rear tires. If that shop also does alignment and suspension work, then you might want to have them give you an estimate on whatever is needed for the rear suspension to be restored to correct alignment.

If that tire shop does not do alignment and suspension work, then I would suggest that you drive the car directly to a garage that is known for good suspension and alignment work, and hopefully they can properly align the rear wheels (as well as the front ones). Bear in mind that excessive wear on rear suspension components might make it impossible to do the alignment without first doing some repairs.

One additional caveat–DO NOT go to Midas, Meineke, Monro, Sears, or any other chain unless you want to risk substandard repair work at inflated prices.

Ditto to what VDC said. The extent of the abnormal wear indicates a problem well beyond a simple alignment deviation. Consider it unsafe to drive until you get it looked into.

… but consider having it towed first to the repair shop rather than first for new tires. If you get it fixed the repair shop can probably provide tires almost as cheaply as the tire shop. And if the fix would cost more than you want to pay, you would not want to have bought new tires.

Thanks for the advise folks. I will make sure to follow your instructions.