I have a v6 3.1 L firebird. It s a sweet car and I take a good care of it. Couple months ago, I put on 2" wheel adapters so I can use a set of 93 z28 camero wheels on it. The tire are 11" wide and they still in good condition. Lately, the car is vibrating when the speed is between 70 and 80 mph and then the speed decrease alittle after 80 mph. I am wondering why is that vibration bcz I never had it before. Any body can help and tell me what might the problem be. Thanks alot.

 

If they are stock 93 wheels then they arn't 11" wide (more like 9"). First off check the wheel balance. You said you got an alignment and rotation, but an out of balance condition is usually the cause for most wheel vibes. If it's not the balance, check your wheel adapters make sure they are torqued down good. At least 85 lb-ft. If that's OK then it's time to check out your driveshaft. An aluminim one from a fourth gen is a good upgrade and the LS1 ones have a vibration dampener.

 

The viberations is mostly from the front. and I can see the dashboard is shakin from the vibration.

 

tire balance or crappy adapter maybe

 

Bent Rim? Tire outa ballance? I would have them rebalanced on a computerized balancer.

 

I also suggest tire balance. By the way, those are 16x8 wheels most likely with a 245/45 tire. The tire width is 9.6"

 

Also when I brake at high speed(70 to 80 mph), the brakes shake sometimes (I guess for the same reason of the vibration btwn 70-80). I had my brakes done couple months ago (front and back w/ front rotors). So I think all the vibration s front the tires.