I’m not sure those mpg numbers are correct: the more fuel efficient Piaggio EVO update, with winglets etc, in the extended range version can hold about 450 gallons of Jet A and has a 1770nm range, so even with plenty of reserves that doesn’t get it anywhere above half of 15mpg (from what I can find IFR reserve comes out to 75 gallons, thus 4.72mpg nautical or 5.4 statute).

Which is why the jump in efficiency would be so extraordinary.

I recall from the last time this plane hit the news cycle that it is designed to cruise at 60,000 feet where air density is lower and better fuel efficiency can be had.

To put that claim into context the manned single engine piston height record is held by a specially modified Rutan Long Ez and was around 35,000ft.

Prop planes don’t work well at higher elevation. There’s nothing for the prop to work against. It’s why jet engines, with their compression, work better.

Props work better than jets at low elevation where the span of a propeller can work against a lot of high density air.

Turboprops are less fuel efficient, they just have a higher power to weight ratio than piston engines.