just wondering, if you have 2 motors that are the same- one turning 40,000 rpm and another turning 28,000 rpm wich will use less fuel..???

I think it will still come down to throttle control mostly but clucth set up as something to do with it as well.

Given there is no other stipulations/conditions, just answering the question at hand, no load differences and same fuels ETC. ETC. the only logical answer is the lesser one.