I run 4.11's with my M6.
Check out this gear calculator.
http://www.f-body.org/gears/
Select 1998 F-Body M6 and it will fill in the tranny gear ratios for each gear.
3.42 at 80 MPH in 6th = 1800 RPM's
3.73 at 80 MPH in 6th = 1950 RPM's
4.10 at 80 MPH in 6th = 2150 RPM's
That is on a 275/40/17 tire.
I still do pretty good on the highway MPG wise. Dropped probably 2-3 MPG.