Best engine thermal efficiency is high load (low manifold vacuum for lower pumping loss) at less than WOT (doesn't activate full load enrichment), and low engine revs (minimum engine internal friction).

With a manual this means short shifting (2000-2500) and using about 3/4 throttle to accelerate.

OEs play some games with auto trans shift points to maximize EPA numbers, which would generally mean early upshifts at less than WOT.

Duke