I haven't bothered to keep up with current standards, but at least in the '70s there was a "break point" at 6000 lbs GVWR. Below that, pickups had to meet stricter standards. Above that was where the Government considered trucks to be more commercial. As a result, there were a lot of trucks and vans rated at 6100 lbs GVWR to evade the emission standards. I do not know when the window stickers showing MPG ratings came into being. I do know my 2012 Tundra, at 7200 lbs GVWR, has a lot of emissions stuff on it. It has already been recalled three times for "upgrades" or "recalibration" of some of it.