I have a 67 firebird 400 turbo 400. When I bought the car, it had the wrong rear end. I put a 10 bolt posi in it. When I did this I had The drive shaft redone. The speedo reads anound 80 at 55. It was correct before the swap.
spinning the rear wheel, I come up with 41 turns on drive shaft to 12 on the tire. 3.41 ratio. I have a dull vibration at about 55 to 60. It gets better at a faster speed or less speed. Seems to be better under acceleration. This swap was done 10 years ago, the other rear end is long gone. I've just put up with it. any suggestions? Thanks, Ed

Hi ed_s
Check the angle of your yoke, make sure it's the correct angle with your driveshaft and trany-tail..
Look for missing weights on your drive shaft.

You can pull your driveshaft and have another shop check the balance on it.

What would be the correct angle and do you have a suggestion on how to check? Do you check with the rear end hanging or with the car on the ground? Something else I wanted to add, The car seems to be running an uncomfortable amount of rpms. It so feels like it needs one more gear. I'm not sure if It's just because I normally have overdrive. I do not have a tach in the car but I did an rpm calculation on line and came up with 2400. That doesn't seem like too much. It feels and sounds like it is running more than that, but maybe I am wrong. Any thoughts? Thanks for the help, Ed