What would cause this???
I have a '94 Cavalier that I just had new tires installed on. When I get up to about 70mph, I get a vibration. It seems to get better at 80mph.(Try driving slower on 696 in MI, you will get run over)I know that the wheels are balanced, I watched the shop balance them. The rear shocks have been replaced.
I also have a '92 sunbird with the same problem, I had the wheels re-balanced on it but it did not make any difference. The sunbird does it much worst then the cavalier. What else causes this problem, it seems to be comming from the rear of the cars.