60 mph = about 2550 rpm
69 mph = 3000 rpm
80 mph = 3500 rpm
If the automatic revs almost 1000 rpm lower in top gear, then that could generate some amazing mpg numbers.
However, in the 5 speed I love to accelerate "vigorously" in first and second and then pop it into fifth at 35 mph. First off the line AND awesome cruising mpg.
Maybe you'll have to trick the computer to shift earlier by accelerating moderately and then lifting off to allow the highest gear/overdrive to engage and then slowly reapply some throttle. Also letting up on the throttle on uphills artificially kept the transmission in overdrive. That technique worked very well with an old car of mine (3.3L V6 Oldsmobile Cutlass Ciera). That usually yielded about 33-35 mpg highway.