I would bet money that the 81mpg car had a special ECU that was tuned for fuel economy and wouldn't pass EPA's emissions tests. For the last 5-10 years, diesel engines have had a really hard time meeting emissions guidelines, so the cars that we drive are tuned to meet the emissions standards. Remember when Mercedes and VW had to team up in a joint venture to meet emissions? If you advance ignition, you get NOX, if you retard ignition you get particulates. I think the ECU's in these cars were tuned for 100% fuel efficiency for some publicity. With common rail technology you can do incredible things, but what is sold today is a necessary compromise of emissions, noise and fuel economy. So I bet these test cars were loud and stinky, but were able to get 81 mpg. It's sad that we don't have the option to do the same to our cars.