[brandall10]

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.