American cars must have different engines, as in most Eurpean and Japanese cars 2500rpm in top gear on a level road will equate to a speed of just over 50mph. Once you start approaching 70mph your fuel consumption rises dramatically, and at over 80 it can be twice what it is at 50.
Mind you, the sort of idiots who think they have a divine right to drive at any speed they like are hardly the sort of people to give any consideration to their effect on the environment.

However, even more annoying than wankers hurtling down the road at 90 with bllind faith in their own superhuman abilities and the capabilities of airbags, ABS and crumple zones are the imbeciles who sit in the middle lane at 60 (or any speed come to that). Many times I've undertaken people who appear to be blithely unaware of the lane on their left. There is no such beast as a 'fast lane' on British dual carriageways; they are overtaking lanes, and the Highway Code states that everyone should keep to the nearside unless overtaking.