Perhaps I'm misunderstanding you, but if you're saying that rpm's raised by inertia don't use more gas than lower rpm's, you're incorrect
Every time that spark plug...uh, sparks...it ignites the air-fuel mixture in the cyl and burns it, no matter how the engine got to that rpm. Leaving the engine in a gear that sustains substantially high rpm's is only practical when ascending/descending a steep grade, the latter more for brakes than anything else.