As a general rule, best efficiency for a spark ignition engine is at low revs and high load without fuel enrichment( which is typically 20 percent richer than stoichiometric except large supercharged aircraft engines of yore where takeoff power may have used as much as 50 percent enrichment with water-alcohol injection). This can be difficult to achieve with a carbureted engine since fuel enrichment generally starts at about 6" Hg manifold vacuum, but on KE and most EFI systems, fuel enrichment doesn't occur until the throttle is to the floor, which activates fuel enrichment via an electrical switch signal to the ECU.

Pumping loss from high manifold vacuum is a signficant parasitic loss, which is one major reason why diesels are more efficient, especially in city driving. At low manifold vacuum spark ignition engine pumping loss is no more than diesels, but most of the time SI automotive engines are operating with significant manifold vacuum.

The spark advance map is also an issue since most maps limit advance at low revs to keep the engine out of detonation, so there is a low speed limit in terms of peak efficientcy that usually can't be determined without laboratory testing.

In most cases, taller gears with an optimized spark advance map that keeps the engine on the ragged edge of detonation is best for fuel economy, and this can be seen in the very tall gearing of modern cars with six to eight speed transmissions that may be set up for at little as 1100-1200 revs at 60 MPH, especially on large displacement engines.

That's why modern Corvettes with 6.2L/450HP engines can knock down 30+ MPG on long freeway trips.

Duke