Vibration 45-50

jpl572103 said
66 Dart, 833od. I have a 3rd/4th gear vibration while cruising, more vibration in 4th depending on how much I push the gas pedal. It's not a revolving type vibration, its just there if that makes any sense.

Has a new balanced 3inch steel driveshaft, has a shortened 8 3/4 with super quiet 3.55 sure grip gears,has diff additive, has dr diff axles. The Vibration happens when I hit 40-45 in 3rd or 4th, worse in 4th, but when I put in Neutral it seems to diminish or go away mostly but when giving throttle it comes back and when cruising it will stay there. Would the shortened 8 3/4 be causing this say if the pinion angle were wrong? I don't think the drive shaft is the issue since the car is smooth under 40 and vibration almost goes away when in N. Also have new Jegs wheels/tires/ balanced.

When I start looking for a vibration the first thing I do is put the rear on stands and duplicate the speed.
If it’s still there I pull the tires and test again.
Then put the front tires on the back and test again.
This mostly eliminates tires as the problem.

This assumes the vibration is not there in neutral at the same rpms.
If it is, then its not the drivetrain but somewhere between the front balancer and trans.

The part that sucks is you can’t trust anything anyone has done.
I had a driveshaft balanced to the tune of $125 before and still it was out of balance.
Also had tires balanced before that ended up being an ounce out of balance.

Unfortunately we can’t trust anything someone else does anymore.