Specifically curious about the Elise:

City ("Urban" in UK) is quoted as 23mpg in both countries.
Highway ("Extra Urban") is 41.4mpg in the UK and 27mpg in the US.

These figures come from Lotus for the Standard US Elise and the 111R in the UK which has the same engine. I know that 41mpg is realistic from having talked to owners who have said as high as 39mpg on long trips. So I'm wondering what's different about the testing?

I must say that I'm a little suspicious of the numbers only because the UK Exige (standard and 240R!) is listed as having the same milage as the 111R yet its (slightly) heavier and has a higer Cd. So if Lotus is just copying and pasting from car to car they might not be accurate. But that doesn't change the fact that ~40mpg is possible.