The other night, a friend asked me what I would do to make a car that got 100-something miles per gallon. This is a perfect example of how that question can be answered in a number of ways. This Prius has been outfitted with new technology LiFePO4 batteries, and has been recorded achieving 125+ mpg.
Having better batteries means it can use more stored power before having to run the engine. While this is an improvement in city driving, on the highway commute this vehicle will be right back to 40mpg. Even the best batteries will be exhausted fairly quickly and the engine will end up running for the remainder of the trip.
Claims that the Prius achieves 499 MPG could be made just as easily and accurately if they measured the mileage on 1 mile test drive on a totally flat road, with no engine start or braking occurring. As far as I know, there is still no standard course for measuring these claims, and most of the time, no one understands the science well enough to question them.
I'm not trying to shoot down Toyota's efforts at making a more efficient car. In a sense, they have done this. However, as usual, press releases never seem to detail the nature of the improvements made, only the part everyone wants to hear. I suppose this is why I never made it in marketing.