I have 4.30 gears with 285/75/16 and cruise 75 mph @ 2500rpm 80 mph is usually about 2700-2800rpms....At 65-75mph i'll average from 16-20mpg depending on traffic.
I just did a 80 mile trip a few minutes ago and my Mileage on my OBDII using Torque app showed 19.9mpg.