Why does 01 Prizm only vibrate at 55-65 mph but NOT at 70-75 mph or below 50 even after alignment and new tires?

Had this same problem for two months. Ended up being the alignment. If you can feel it in the steering wheel the problem is probably up front. If you can feel it in the seat then it's probably in back. Cost me $90 for front and rear alignment combined. Look at the interior wall of your tires, check for uneven wear. Changing the tires alone won't fix it; you'll just destroy your new tires.