Still not following. The only variable in your example will be the engine RPM. Drive shaft RPM and drive wheels, and thus speed will be a constant at 3K RPMs - i.e. transmission gears will only effect the engine RPMs, but 3K RPM at the drive shaft will always be the same speed regardless of what gear it's in.
Maybe I'm missing the point of this test.
First, thanks for being kind.
With the car MOVING, the car's speed WILL be determined by the drive shaft RPM, the rear gear ratio, and the tire diameter. I always knew that but just "old guy'ed" the concept in my first post.
IF one lifts the drive wheels off the ground, and rotates the drive shaft, the drive wheels will be rotating at a rate commensurate with the speed (MPH) the car WOULD go, but the car goes nowhere; 0 mph. That 0 speed isolates any 'speed factor' vibrations (suspension, wheel/tires, etc) from the experiment (not necessarily from the problem). What's left is ONLY the contributions (if any) from the rotating drive shaft.
Now, because I'm going thru this in my head again, to completely isolate all wheel/tire vibrations, the rear tire/wheels need removal. They WOULD spin, and may 'vibrate' if unbalanced.
When driving the car and 'feeling' the vibration, the poster stated it started around 60 mph. If he was in high gear, and that ratio is 1:1, whatever the tach reads at 60 mph, is also the drive shaft rpm upon vibration inception. I used 3000 rpm in my example because it made the math easy.
I went on with how one could 'measure' 3000 drive shaft rpm by using the engine rpm and a M20 transmission (as an example). Third gear is approx 1.5:1. Or 4500:3000. 4th Gear is 1:1. OR 3000:3000.
Said another way, (in my example) the car does 60 mph at 4500 tach rpm in third, and 3000 tach rpm in fourth. Drive shaft rpm is 3000 for both.
Am I making any sense?