You are aware I presume that the UK gallon is not an official measure but is taken as 277.4 cubic inches whereas the US gallon is an official meassure defined as 231 cubic inches. This means that the UK gallon measures 1.2 US gallons which kind of explains the differences in mileage you're quoting.

Hey guys my BMW 325 convertible at best get 23 mpg. I don't care how slight a difference it is in imperial versus ours the results don't lie. In 1992 in Germany I could have got the vert in turbo diesel but I didn't know it. He's right the less tax dollar they get the more they get ticked off.
As an American vet I'm tick off that the govt. doesn't let these cars in our towns. I think more people could go on a vacation and spend more money if they could save it somewhere else, that means spending more money when you're gone. Our leaders do not live in a world like ours. They fly around in aircraft thinking they are rock stars never remembering they are public servants. They work for us. How about taking care of us instead of paying foreign lands to be our friends. 1 billion to Egypt, 60 million to Jordan, 40 million to Palestinians. If they want to be our friends then quit paying the lunch money bully to be our friend. They have the oil let them pay us. In 2016 Abu Dhabi runs out of oil and starts relying on natural gas sales for their revenue. It's time to stand on our own energy legs.

The other issue is the food for fuel agreements the USA entered into after WWII as part of reparations and compensation to Europe and the Middle East. Yes, the USA helped the allies win the wars, but the USA was also stuck with Britain's debt, reconstruction costs AND high taxes on everything, including the highest fuel costs to offset USA military being the largest world consumer of fossil fuels (Marsh,2010). So while UK economizes, the USA pays and pays some more to ensure the Middle East keeps pumping out petrol - and while its doing so , continues to dessicate regions of Africa and the middle east that otherwise were once verdant and productive. Its all a form of indirect economic and environmental leveraging to preserve cold region populations at the expense of warm weather populations, top the point polar ice caps are melting, massive storms and flooding,tsunami waves, all these climatic changes are displacing outlaying native people from their islands and coastline ways of living.

Military/gov, in keeping with "food for fuel agreement " (Google that) have long suppresed high mileage, efficient carburation systems:

“I have been able to obtain extremely high gas mileages with the system of the present invention installed on a V-8 engine of a conventional 1971 American made automobile. In fact, mileage rates in excess of one hundred miles per gallon have been achieved with the present invention. The system may be constructed with readily available components and technology, and may be supplied in kit form as well as original equipment.” - Thomas Ogle Inventor (from second to last ¶. Retrieved from http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=4177779

Marsh, D. (2010, June). Our LETHAL Dependence on Oil. United States Naval Institute Proceedings, 136(6), 48. Retrieved from Research Library, (Doi: 2061054711).