The difference would seem to be consumer protection laws in the USA
Lets look at the MINI published numbers in both Countrys
First number is the published mpg in miles per US gallons, in brackets are these converted to mpg in Imperial gallons. Third are the claimed mpg in the UK in imperial gallons.
These numbers are for the Countryman Cooper S (petrol) as the diesel is not sold in the US. Highway equates to extra urban, City is urban and combined is the same.
2 wheel drive.
Highway 32mpg (38.4 imp) UK 52.3
City 26mpg (31.2 imp) UK 37.7
Combined 29mpg (34.8 imp) UK 46.3
Highway 31mpg (37.2 imp) UK 48.7
City 25mpg (30.0 imp) UK 34.4
Combined 28mpg (33.6 imp) UK 42.2
Yes there is a difference in the cars, US are quoted as being 181 hp and UK as being 184 hp. Which would not seem to be enough for the huge difference.
So it really does look as though Mini are forced to be honest in the USA due to consumer protection laws But perhpas not so honest in the UK?
From what I've read on here owners seem to be getting actual consumption similar to the US figures.......