What are the possiblities for my car ('88 V6auto firebird) to shake at speeds over 70mph through 95+mph? It doesn't shake so much that i won't go that fast, but you can definitely see your chest and the dash vibrating while driving down the highway.

 

check to see if the wheels are balanced, driveshaft balanced, alignment?

 

if its not tires and such, check out your tie rods, outter and/or inner, mine does the same thing, don't worry though it starts at 71 but after 100 or so it smoothes out again

 

^ I have the same thing w/ my 86 Berlinetta....how hard is doing the tie rod ends?

 

i would look into the driveshaft 93-97 camaros had a big problem with that

 

1. Jack up rear of car

2. Put on stands and chock wheels

3. Start car, put into D or OD any run up to 70-80 MPH. See if vibration is there. Turn off engine, apply brakes and try to avoid putting shifter into park.

4. If no shaking, then it is the road causing it. If it is there (the most likely case) unbolt your wheels, tighten the lugs nuts down to keep the drums in place and repeat #3.

5. Remove drums and repeat #3 EXPECT DO NOT TOUCH THE BRAKE PEDAL.

6. If vibration is still there, remove driveshaft and get it rebalanced.

 

The stock drive shaft on your car will still start to shake rattle and roll no matter how well balanced. You should find another one from a 4th gen donor or buy a new aluminum one. Buy a carbon fiber driveshaft and 0% vibration all the way up to 200mph
an aftermarket Aluminum driveshaft is good to about 150 mph without shaking your teeth out of your head like the stock one will do after 95 MPH.