Vibration ONLY at 75 mph

Darter6 said

This is just a thought..Harmonics.It was explained to me by a airplane mechanic years ago,, something about engines develop a vibration at a given rpm during flight. I don't know much about it but something that was addressed during WWII .Maybe google it ???
Again just a thought.

Click to expand...

Remember the first statement pleeeease, read the entire first post, the vibration is present while coasting WITH the engine off, as in not spinning.
But thanks for the idea, if the condition was related to rpm it would change with different gear ratios as the rpm changes, but it has nothing to do with the engine, plus the engine was formally in my 66' and it was dead smooth.