Originally Posted by andrewwynn

15.068*3=45.204. That is very likely the spike over to the right. Getting a distinct vibration at 3x wheel RPM, something is vibrating close to that harmonic.

39*7.3/3=94.9 feet/sec.

65 mph should make the wheel RPM 1/3 of that harmonic see if the shake dials in at that speed. It's possible the shake is just more annoying to you at 75 because higher amplitude etc. If the spikes align at an exact multiple of tire rotation it's definitely tire/wheel related.

I would change the prime suspect with spare and move that around. You might have thrown a wheel weight or have an internal belt delamination issue (that happened on my Z28.

A little shake finds its way into the un sprung suspension and bingo: harmonic