The maths are about right for MPG. Time is what you (might) save. I'll get around that at cruise - 135kts or 155mph @ ~ 10gal/hour. Better fuel usage if you slow down a bit. Tempted to say if you slowed down to max Prius speeds on the autoban, the C182 might be more fuel efficient. I'm sure it gets 50mpg at 75mph, but doubt it gets that at 109mph.