our (Freightliner Columbia MBE 450/1650 hp/tq) is equipped w/ the 12 speed Meritor ZF Freedom Line SmartShift and a 3.21 rear...and at 70 MPH we turn just under 1500 RPM.
at 65mph we're turning 1300 RPM
Although I havent recorded the RPMs at 55mph I know its a sweet spot for MPG (13.3-ish from...