I have always had a vibration while driving my 71road runner. It really starts to become noticeable around 45 - 50 mph. At 65 - 75 it is quite pronounced and rather annoying. If I ever take it up to 80 -85 it feels like the car might come apart. I finally said what the hey and wanted to push the issue and ramped up from there. All the sudden at exactly 110mph the vibration totally goes away and becomes so smooth and quiet I thought the car shut off. (actually scared me) I never took it any higher but as soon as I back it down from 110mph Mr. vibration is back. I have changed the tires all new balance and such, but no change.
To get specific the vibration is much like a slow rolling drone that just repeats. Similar to the sound /feeling of flying in an old WWII bomber with those big 'ole radial engines.
Do any of you guys out there have a similar situation or possible know what might be causing this? I am not really sure if it comes from the motor, drive shaft balance, or what. However, as best I can tell I don't feel the vibration just holding the gas down to simulates revs at say 70 mph while sitting in the driveway. I gotta tell you this car is so smooth and fun to drive at 110 mph without the vibration but we all know that is not possible. Any help getting a solution to this problem would be greatly appreciated.