Okay - Apparently there were 276m vehicles registered in the US according to this
https://www.statista.com/statistics/183505/number-of-vehicles-in-the-united-states-since-1990/

The average is 24.9mpg on the US registered vehicles fleet but I don't think this is necessarily representative of the real world given that the US tends to have more older vehicles which would drag it down a bit.
According to the US EPA https://www.epa.gov/automotive-trends/highlights-automotive-trends-report.

I did not spend long enough to see if that included all the trucks (there are LOTS of those) buses and big stuff in this number.
Love those old 15mpg Oldsmobile's and Cadillac's, maybe not on UK roads.

So at that point I bail rather than try and draw any conclusions. Even if I take my socks off and borrow the good ladies fingers and toes I am not going to be able to count that high tumbleweed

Everyone loves a Morgan. Even me, unless it's broken again.