I didn't bother with the entire video at the link, but they guy tells you what the whole story is in the first few seconds.

The UK, and most Euro countries, have far less stringent emission limits than are in effect in the US, and the spectacular fuel consumptions he reports are for diesel fueled vehicles that can't be obtained or operated legally here.

US manufacturers build some of the engines used in Europe, that do get the mileages quoted, but can't legally sell them here. Vehicles made in Europe that get the super consumption can't legally be imported to the US by dealers. Although it's possible for an individual to "bring one in" it generally requires modifications and lots and lots of paperwork, and is seldom worth it - and the modifications required to meet emissions requirements here generally negate any fuel economy advantage.

One factor affecting what gets used is that the majority of European countries rely largely on imported fuels, and have difficulty just getting sufficient quantity of crud(e) to refine. They have not imposed restrictions on diesel sulphur emissions as are the rule in the US, and also - due to smaller quantities used - can pick from the few sources that have lower native sulphur content.

Despite all the whinging about imports, a fairly high percentage of fuels used in the US are from local sources that, unfortunately, have higher sulphur than what's used elsewhere, so additional devices and methods are needed to clean up exhausts when you burn the end products - whether diesel or gasoline.

Higher combustion chamber temperatures generally give higher efficiencies, but also increase SOx emissions, so diesels in Europe generally can run at higher compression ratios (producing higher combustion temps) than those in the US.

The same relationship between compression ratios and efficiency are mostly true for gasoline engines, but US regulations mandate that the majority of small vehicles must run on "regular" gasoline (fairly low octane) with strict limits on additives that could raise the octane numbers. This allows using much more of the easily available lower-quality crude that the US has, but that is a smaller fraction of the fuel stocks (crude) that the UK/Euro regions can get easily.

If Europe could use the same crude stocks as are common in the US, they would be forced to regulate emissions in order to do so (without killing their people), much as the US has done, and would no longer be able to obtain the "km/l" performance they currently enjoy.

In each of the separate regions, the fuel economies obtained are due to different compromises intended to make best use of the differences in resources available, and in both regions the successes obtained have been pretty good, but they can't be interchanged or even compared simplisticly.

John