There is this:
The Shell Oil 1973 Test Car
376 miles per Gallon
0,625 liter/100 Km 1 liter/160 Km
You can read about it here:
Shell Opel
As you said "200 miles per gallon if you're traveling 8 miles an hour but it is not really reasonable"
This was a extreme test, the car was highly modified to go just about 30MPH ONLY, and it took a bit of time to get to that speed with a over heated engine rigged to run off a small lawn mower/ or small motor cycle carb.
And this test was a flying start, that is it hit the traps running at speed and only for a short mile or two miles for the test and I think some nypermileing or coursing was involved.
I am surprised they did not find a high hill and let it idle as it roller downhill.
It was a totally undriveable car in any real world.
BUT THIS one was a real world stock cars and they were driven on real roads test:
An English TV show Top Gear Series 12 Ep. 4 Summary: This week's challenge is to drive from Basel in Switzerland, to the north west coast of England. This must be done on just one tank of petrol - no splashing and dashing in France, just one tank. The first to arrive - if any do - gets the honor of switching on Blackpool's illuminations. Some 740 Miles. Air Date: November 23, 2008
So one car is a Volkswagen Polo Blue Motion, a Diesel 3 cylinder that is rated at 75MPG!!!!! and has a 10 Gal. tank driven by Richard Hammond. The second car driven by James May was a Subaru Legacy diesel rated at 50MPG and has a larger tank lastly. Jeremy Clarkson shows up with a Jaguar XJ diesel, this is a full sized car with four doors etc. His plan was to run out of gas near London and thus go home, it was rated at 35MPG.
What happened?? First ALL three cars made the 740 Mile trip!!! What is even more insane was at the end of this show Jeremy Clarkson whom did NOT drive sanely until he had passed London was very surprised the Jag made it but wait there is more, he reported the shows engineers checked how much gas was left in the tank and reported that the Jag could have gone 260 Miles MORE that meant the Jag had a range of 1000 MILE per tank.
Point of Order: The UK Gallon is not the same as the US Gallon, so a little calculation shows that would ONLY be around 850 MILES per a tank in US Gallons of gasoline.
By the way none of these cars had any areo done to them.
From what I have read right here on this very site AREO is only worth a few MPG even in the extreme, as in removing side mirrors, windshield wipers, door handles, rain gutters, roof racks, then adding under belly pan, rear wheel fender skirts, if possible front fender skirts if not then large disk like wheel covers, and lastly insane Boat Tails.
And after all that you might gain 5 MPG.
So check out the current Big Rigs, ground effects, a curved air defectors on the cars to push air up over the boxes and the boxes with some under belly deflectors.
That is it. Truckers will fight to gain even .05% more MPG.
And that is as far as they have gone.
Rich