Just watched Top Gear where Jeremy Clarkson drove a 200hp Jaguar XJ 2.7 V6 Diesel Twin Turbo to a theoretical 1000 miles before emptying the tank of fuel. Meaning that he was doing an average 44mpg. That's a full size car we're talking about. As for pollution, it makes 40 to 80 grams of carbon dioxide per kilometer less than all the other V8 Jags. It it does double the mileage, pulls a decent 200hp and pollutes less than the other Jags. Oh, and it costs only 4000$ more than the base Jag.

Too green for us probably. Ecologists tend to not like real green cars. No profit to be made.