I know why cars get such terrible gas mileage in America!

Roundabouts!
That's why American vehicles have such awful gas mileage. It's not that they're any worse than any other vehicles, it's that the cities here are laid out such that even in the suburbs you can't drive more than 200m without coming to a stop sign or a set of traffic lights. The constant stop-start driving destroys fuel economy, and could be so easily rectified with roundabouts.
Some driver training would be required though. I hate coming across roundabouts here because the average American driver has no idea what to do with them. Watching Americans trying to negotiate a roundabout is an entertaining and embarrassing representation of their ignorance about an intersection that does not require you to stop but does require you to yield, but only in certain circumstances. It seems to cause some sort of severe psychological disorder since it is neither green nor red, but requires them to steer and calculate approaching distances and speeds at the same time.
Having said that, roundabouts are, generally speaking, a great idea. Unless you do them like they do in England, where the roundabout cures the traffic problem so the local council then puts traffic lights ON the roundabout to bring the problem back. I'm looking at you Berkshire county council.