It just depends on the combo. My 11:1 360 with a big roller cam got better MPG at a 3800 rpm cruise than it did at 3000. I did 4 drag weeks with that 360, the first year we had to run down I-40 and I had to run fast to not die in traffic, 3800-4000RPM for 100 miles netted just under 15 MPG, it usually got 13@3000-3500 cruise. Getting close to peak torque and getting to the tighter side of the converter is the reasoning I am sure.