This is nonsense.
http://jalopnik.com/5981938/why-do-european-cars-get-better-mpg-than-us-cars

Even adjusting for the different gallon it still means that the US version TDI Jetta only gets 54 mpg as opposed to 65 in Europe. Some of that difference is down to the testing. The average car size in America is larger as well so that skews the average figures against the US.

That's just not really true anymore though.
Car models are more or less uniform across markets, and whilst smaller cars might be popular here, a BMW or a Chrysler or a Jeep or a GM mid range 4 door is the same size here as it is there.
The era of the UK pottering about in little Austins has gone; if you hadn't noticed all cars a big today. My Camaro is narrower than a Merc A Class.

Yes but the Americans drive a lot more trucks like the Ford F150. About cars in general you are right. They are made for a global market and so there few differences between countries. Europeans drive a lot more tiny cars like Smart cars and Fiat 500 etc than they do in America, but I think that is changing with the $4 gallon in the US. Toyota are once again the worlds biggest car seller and its prius camry and corolla are selling very well in the US.