Yeah, but it's 50 years old. 12 was the average, and you really didn't see anything getting better that 25. That's on older roads with leaded gas and bias tires.
I'm getting 6.7l/100 km using Google. Not sure what math you're doing. But keep in mind there are two different gallons, US and Imperial or something. 35 mpg is pretty normal for a gasoline-only car these days (hybrids can do better depending on driving conditions - if they can spend most of their time on battery, for example). Diesel cars aren't much of a thing in the US, again, especially after the whole Volkswagen cheating thing.
I know there used to be diesel cars in the 80s that would get crazy numbers like 50 mpg (4.7l/100km), and gas-powered cars probably could now, too, but emissions and safety requirements have pretty much killed that.
Yeah, doing the math I also get 6.7L/100km (assuming US Gal, which I presume is correct since they're using mpg). Also, looking at Wikipedia, it looks like the testing standards between the US and, say, the EU, can cause potentially a 1L/100km difference between the two, where the EU standards give a lower L/100km.