Can't say that I'm up on EPA tests or ratings, either. All I know for sure is that our mileages in late model vehicles generally exceed their ratings by quite a bit. Of course, we really don't know what any city mileage would be.
Back-in-the-day cars, huh? To me, those didn't really have any emission standards, and definitely no computers. Basically, I went from (mostly) Sixties and Seventies vehicles directly to the '18 'Cat, bypassing all the mediocre stuff in between. Or put differently, from easy to work on vehicles to vehicles I can't work on.
Either way, other than maybe steam engines, they're not meant to idle. Heat dissipation (hot spots), poor cylinder sealing, and other issues seems to be the reasons. So, since I try to be nice to my vehicles they aren't idled. Case in point in case I didn't already mention it, my '21 Ram 1500 has 1 idle hour on it. Yes, one.