There are other things that may affect MPG, like mixture. Some states change the mixture of their gas depending on the season, some places like Cali have MTBE others like Illinois(?) have ethanol and who knows what else is getting mixed in there that kills efficiency.

I also read a blurb in the latest Car & Driver about Isuzu's new 3.5 V6. They said that the sulfur content of American gas prevents it from getting either ULEV and/or more efficiency, I forget exactly.

So, just another couple of possible factors that might apply, especially when comparing UK gas to American gas.