The main reason European vehicles have such low official consumption figures is that the European NEDC driving cycle (used for the test procedure) is even lamer than the EPA's test procedure, *and* the EPA arbitrarily applies a fudge factor to (supposedly) more closely align the window-sticker numbers with reality, while Europe does not. That, and a "gallon" (which is only used in UK! everywhere else uses L/100 km) is different than in USA.
Tuning differences and emission control differences between European and North American versions of the "same" engine are insignificant with regards to their effect on fuel consumption. The technological methods by which you achieve compliance with Euro 5 emission standards are pretty much the same as those by which you achieve compliance with US EPA Tier II standards, even though the testing procedures are different. Gasoline engines have EFI, three-way catalyst, narrow-band O2 sensor, EGR, etc. All that stuff is the same. The calibration differences are insignificant. Diesels have slightly different emissions hardware, but the difference that it makes on fuel consumption is insignificant. DPF (and its regen procedure) across the board, EGR (there are slight differences in hardware and calibration) across the board, oxidizing catalyst across the board, newer models mostly use an SCR catalyst requiring AdBlue across the board. Minor differences in calibration but that's it.
If you take an actual vehicle from the showroom in Europe and the comparable model from the showroom in North America and drive both of them the same way, they're going to use very close to the same amount of fuel, even though their respective window stickers are very substantially different because of the differing test procedures, size of gallons (if you are not using L/100 km!), and lack of fudge factor in Europe.
