Mileage figures have little to do w/ max power. That mileage figure is roughly using the same actual power, which is a relative sip of fuel.
The main difference would be in testing EPA methodology which which would be a bit more stringent/realistic to world use. On top of that, there likely is more rolling resistance at lower speed due to larger tires and heavier weight, offset somewhat by lower C/D at speed.
Basically it's hard to extrapolate ICE efficiency gains... they're there are sure, but probably in the single or low double digits.
The main difference would be in testing EPA methodology which which would be a bit more stringent/realistic to world use. On top of that, there likely is more rolling resistance at lower speed due to larger tires and heavier weight, offset somewhat by lower C/D at speed.
Basically it's hard to extrapolate ICE efficiency gains... they're there are sure, but probably in the single or low double digits.