[P-hack]

semi trucks have tons of gears, and live with low acceleration. If you want best hiway mpg, you size your engine for hpNeededForCruise=peakBSFCHP and gear accordingly, and live with whatever acceleration you get. There will be reserve power beyond bsfc. If the engine is "right sized" for most efficient cruise, then yah more gears would be better.

A larger engine will fall away from bsfc peak on rpm and/or load, you could probably make an educated guess how much even without an actual bsfc map, i.e. if a 250cc is just right for bsfc cruise and getting 225gm/kwh, a 500cc at that hp and rpm is getting very roughly 300gm/kwh (load cut in half), or at that hp and load is getting 250gm/kwh (rpm cut in half).

So very roughly a 10-25% loss doubling the engine size away from "right sized" for cruise. Depending if you cut the rpm in half or the load at cruise in the larger engine.