If you go to www.FuelEconomy.gov you can get a personalized report on your own fuel use, compare government MPG ratings to personal reports, and even look at estimates for your carbon impact and petrol consumption. It's pretty neat. I especially like the crowdsourcing opportunities to compare mileage to individual reports. There are some kinks to be worked out (it says my car is a subcompact right now, maybe subcompact for a boat).
More importantly by 2013 all new cars will expected to come with this label which which shows average MPG in especially large lettering along with a quick EPA snapshot, average money spent on fuel, and how your car ranks on all these things. Kind of like calories on menus I wonder whether this kind of enforced labelling actually results in lower consumption.