New to the forums
 

Just got a 94 z28 a month ago and decided to join a forum. Car is basically stock for now. Lt1 m6. Having a issue with high speed vibration been looking all over the Web and it seams it could be a few different things don't want to just throw money at it and not get any where. What seams to work normal to fix the vibrations?

could be a wheel out of balance
could be a tire out of round/bad wheel or unevenly worn tire (cupping) which of course denotes another issue
could be the convertible top and wind getting underneath (if rag top)
could be an out of balance drive shaft
could be a bad u joint
could be a tie rod or other suspension item
could be wind getting "caught" on something
could be the "frame" (unibody) is tweaked and not tracking straight

and the list goes on

you need to ascertain what it "feels" like and go from there. getting your tires/wheels checked/balanced is an easy first step

My speedometer is off so id guess around 65 I feel a vibration and a rumble sound from the car, hard to say where from. If you keep the revs low it seams to help. I rotated the driveshaft 180 yesterday to see if that would help. Didn't change. The front tires we just balance and it has new tie rods. U joints seam to be fine.

??how many miles on it manual tranny auto ,I hate to say tranny ,with the speedo off,welcome to the site .you will get there an figure it out

It's a manual and the spedo is off due to a faulty cluster the tach nor fuel Guage work correctly. The transmission seams fine. Vibration is in 4th as well as 5th and 6th so I don't think it's the 5th 6th issue the t56 gets. I'm hoping a new driveshaft will fix it.