Im not positive, but ive heard it has to do with the additives the us goverment has them put into the gas. It reduces the mpg while supposedly decreasing the polution.

Or possibly its the add ons to the cars the goverent demands

Another possibility is that americans want more on/in their cars, that people in the uk dont demand.

beyound those possibilities i dont really know.

the more man learns, the more he realises, he really does not know anything.