I have a 1986 Oldsmobile Cutlass supreme and it shakes at Highway speed (around 70-80). I have had the shocks and back bushings replaced and my tires balanced and it still shakes. It feels like it's coming from the back and I can feel it in the dash and floor. This all started when I replaced the old tires(245/60/r15) with new ones (225/70/r15), they are taller but I would think that would be the issue since the stock tires were also 70's. Any ideas where to look?
Turns out the previous owner dented a rim and hammered it back into place. It makes sense why it started vibrating when I got new tires in it because they took off all the weights on it that were meant to balance the damage and put on what they thought was enough weights. Either that or when I got new tires they damaged my wheel and tried to fix it. Either way I bought a rim from a junkyard and it works just fine.
Thanks for reporting back with the fix!
If you're pretty sure the shake is coming from the rear ((giggity)) AND this isn't a broken belt in one of the tires, maybe rotate the rear tires to the front, keeping them on the same side.
If the shake doesn't follow the tires to the front, then maybe you have a worn axle bearing back there.
But you say this issue began when you replaced the tires. So I can't see how a worn bearing wouldn't have been acting up before you got the new tires.
Is it possible the shake isn't starting from the back?
Maybe consider taking it in for a front end alignment (wheel alignment) for 2 reasons.
1) It may remedy the high speed shake (and will make those new tires last longer).
2) The guy will check the steering/suspension components for any "play" that's out of acceptable spec and will inform you of the necessary part(s) replacements before he can do the alignment.
(I'm a big fan of, "why not let somebody else do it?", if it doesn't cost any more
It sounds like the new tires are unbalanced. If it was fine before the new tires that would be the logical place to start. A wheel weight could have fallen off after the tires were balanced (if they actually were). Also check the inside of the rims for dirt and clean them if necessary. I live in the rust belt. Here its pretty common for tires to come out of balance during and after the winter months because of the excessive sand on the roads.
I have had the tires Balanced 3 times since this has began, one of them last week, two alignments as well. I have yet to rotate the tires to the front to see if that is the issue but nothing has worked yet. It seems every time I have replaced something that has todo with the suspension it just gets worse.