I had a similar problem when I decided to open my Fiero up one day. Driving down the highway mid day (during work hours, so very few people on the road). First of all, when doing 70, my car was shaking quite a bit (not brand new tires, and probably needed an alignment...but they were low profile high speed rated). When I got to 75 the car smoothed out and there was no shaking. I started following another car that was doing 85 (according to my speedo), and he got over (must have not liked me behind him). When he got over I decided to open it up. I was accelerating at about 10 mph every 400 rpms and every 3 seconds. i got the speedo back around to and a little past the 0 (85 mph speedo in an 85GT), and still had about 1200 rpms until red. I calculated the speed using rpms and it came out 115 mph. Then I tried calculating it on the speedo itself and also decided that it would be about 115 on there, if I continued the dashes all the way around. My car is a stock 85GT with the auto and a shift kit. I later found out when buying new tires for it, that the tires on it were smaller than the original size. My guess is that they were about 1" smaller in diameter than the stock tires. So, if I were a betting man, I would say I was doing about 100 mph, but at that rate, I could get my car up to about 125, and I didn't think the autos were supposed to go that fast, especially since the manual GT's were rated at 125mph.

I'm assuming that when my car got smooth at 75, I was getting that same air pocket under it. Other than that, it seemed very stable at those speeds.

Today driving down the highway at 70, with new tires and everything, my car would shake uncontrollably and then smooth out, and then shake uncontrollably and smooth out again. I found out that my right front mount from the riser to the wing had broke through the fiberglass, and my wing was really loose in that spot. Could that cause it to shake a lot. I've also been getting a lot of grinding noise from my back left tire when swerving to the right. I thought it was brakes so I replaced them to no avail. Then I figured maybe it was a bad bearing, but my hubs don't even get warm. At 70mph with bad bearings your hubs should get hot shouldn't they? Could there be something else making this noise that's also causing the shaking at highway speeds?

quote Originally posted by revin: .planted on the road @160mph

Pure speculation....

i have stock springs, konis, poly bushings, hoodvent, stock tires(bfg traction ta) on my 88, no wing. my caster was set back for tight turns and lower speeds but at 120 i also had vibrations thru the steering wheel and the backend shook alot. this leveled off at about 100+ or grew so great that it blurred togeather. i realigned the car to toe in/caster full forward and it helped a bit but ill agree the fiero isnt really designed for it aerodynamiclly. lowering might help but a new nose and a rear wing should do the trick. perhaps there are already noses and wings out there that can help in 70+ mph.

also the bushings on the fiero are small and the suspension is tight, the steering rack is low placed, set forward, and bolted to the subframe, metal to metal. the stock front end is higher than the rear(dont know why) which probably contributes to the whole problem. perhaps the fins that the evo has on its roof, just before the rear glass, supposidly they pull more air over the back and increase rear downforce.