i was wondering if anyone else has had this problem... my grand prix vibrates while i'm driving. the person i purchased the car from said it was the "performance tires". is he full of it or would the "performance tires" really make the car shake? i get scared when i hit around 80 and 90 mph. i feel like i'm losing control of the car it shakes so bad. any tips would be very helpful. butchrandom@yahoo.com

The only way "performance tires" would make your car vibrate or shake is if there is something wrong with the tire or wheel combination.
Badly cupped tires. This would be evident in looking at the tires.
Out of round tires. Not so easy to detect.
Damaged tire. Not so easy to detect.
Damaged wheel. Again, not always easy to detect.
Suspension damage or wear, alignment problems, can cause vibration or shake
Ask around for a good tire shop and alignment shop and have things checked out by a professional. The problem you are having is not normal and could be an indication of an unsafe condition.

Charles

Suggest that you check ALL the tires. Could be problem with steel belts in the tires. The back ones will make it do that even worse than the front. Had the same problem. You've been given garbage regarding the performance tires. If not the tires, get it into a front end shop where they can run the tires on the car to a high speed. Do not 'mess with it' much longer. That front end steers, brakes and drives the car and you are in for a 'world of grief' if anything happens.

Does the vibration happen at low speeds, or just highway speeds?
If it's noticable at speeds close to and above highway speeds, you may just have to get them balanced. Check to see if there are any wheel wieght on the rims.
They don't have to be on the ouside of the rim, be sure to check the back side. Even then, if they have been remounted, the weights may have not been adjusted properly. This would be the place to start, as it would be the least expensive fix...If that's the problem.
Later now,

Mine did this. It turned out to be a bad alignment. The car didn't pull or vibrate until the alignment had ruined the tires. Most shops will check your align for free. After you get the alignment squared away then have tires inspected and balanced. They're probably bad. I got BFGoodrch 80000 mile tires and it rides better than it ever did with the goodyears, even when it was new. It also handles better and is more responsive than ever. And this is at 100,000 miles. I'll never have Goodyears again!

I have a 98 GPGT, and mine doesn't shake when I go fast, like around 80, but mine shakes when I go slow, like backing out of my driveway.