[MetroMPG]

Your suspicion is right: some cars are much easier to hypermile "out of the box" than other cars. I'm only talking about relative improvements compared to the EPA estimates.

A simple example is straight highway cruising. Drive over about 100 km/h (60 mph) in a 3 cylinder Metro 5-speed in warm weather, and you won't be able to achieve its (old) EPA highway rating.

My mom's Camry, however could be driven up to 112 km/h before the mileage dropped below its EPA highway estimate.

So it's not a level playing field.