My 67 coupe runs great until you get to 60 MPH and then the vibrations start. At first I thought it was the tires but I had them balanced and the vibration is still there. The steering wheel does not vibrate but where I notice it the most is in the rear view mirror. I know Mustangs are not know for smoothest rides but this is wierd that it starts right at 60 mph. Anyone have any thoughts on where I should look now?

 

drive shaft

 

My 67 does this. My alignment is off, that might be it.

 

http://www.aa1car.com/library/vibrations.htm
I had a viberation about 60 and it was a tire that was defective, actually both was over the limit. I had them road forced and found that one was out like 140 lbs! Once changed, it runs smooth as silk. Hence forth all my tire/rim will be road forced (balanced with weight one them like they were on the road rolling). The machine also checks rim runout and roundness.

 

Tire roundness can cause big issues that will not ever be fix by balancing. I had a set of BFG ATs on my Bronco that shook terribly. I finally had them shaved and trued up. Boom no issue. It is not a cheap thing to do and there was only one place in town that did it...mostly for big off road guys. Something like that could also be an issue.

Honestly my car shakes quite a bit. I just chalk it up to being a classic. A 40 year old car is never going to drive like an 08 Accord.

 

The drive shaft is the first thing coming to mind on a stock drivetrain.

But if you've swapped things around, like a 5spd install, a different mill or even lowering...
Anything that would change your pinion angle towards more of a straight line will cause vibrations for sure.
You cannot have everything on a same plane from the engine (crankshaft) to the pumpkin yoke. You've got to have a slight V somewhere.
That's why motors are always pitched in the engine bay, to avoid that straight line all the way through.

Having said that, week bearings and anything that spins throughout the car, have to be looked at as well.

 

I have the same prob with my early 65 FSTBK but not all the time just every so often maybe tires or driveshaft

 

does the vibration get worse as soon as you let off the gas? -Gun