> Plus, 70-75 MPH means 4500-4800++ RPM and
>things get very noisy and "buzzy" at that point.
I calculate that if you have a 3.89 rear (as you should) you should only be turning 4250 RPM at 75 MPH. (based upon 3400 RPM (at) 60 mph).
Looks to me that your tach is out of whack, or you have a 4.22/4.55 rear end in your car.
BTW, Put in the Alpine GT 3.70 rear, and that drops to 4040 RPM at 75.
Jarrid Gross