Tires: Excessive cupping on rear tires, inflation pressure, rear tires

Question
My 2001 Grand Prix (150k miles) has been developing a progressively worse vibration when I accelerate.  I did a walk around and the only visible problem I can see is that the both rear tires are cupping on the inside. Struts??

Answer
Jim,

This is caused by misalignment and aggravated by insufficient tire rotation practices and insufficient inflation pressure.

First step is to fix the alignment and then rotate your tires every 5,000 to 8,000 miles and check your tire pressures monthly.