So some of the manufacturers are manipulating their vehicle's performance to meet the EPA test standards, while not doing so in the real world? Seems like VW got caught doing something similar not so long ago.
The EPA test is standardized and VERY specific. It's run indoors on a dyno, and then that result is plugged into a formula, essentially, and corrected to make a "real world" number to account for weather.
When I say very specific, I mean extremely specific. As in, start at zero MPH, accelerate to 34 MPH
in 8 seconds hold that speed for 14 seconds, then accelerate to 50 MPH in 6 seconds, hold that speed for 47 seconds, etc. I used to have the speed / acceleration / time chart but I can't find them now so I'm totally making those numbers up, but you get the idea. Not only is it specific as to speed vs. time, it's specific to acceleration vs. time.
Manufacturers have tried to game this test forever. They employ specialized drivers that are experts in absolutely nailing the speed / acceleration / time chart to the second. IN the end, it's a standardized test that has nothing to do with how people actually drive, so as they say, Your Mileage May Vary. But it's all they can really do, short of running each car on a road in a closed environment.
VW's scandal was on emissions for their diesel engines. They programmed the computer to recognize the sequence of acceleration / time present for the EPA test, and it switched into a low emissions mode which gave lower real-world performance. Since performance is not measured during the test, it really goes unnoticed. When the ECU did not detect that it was performing an emissions test, it defaulted back to a higher-polluting, but better performing, mode.
I have to say, I own one of the VW diesel engines. And while I was always a little suspicious at how long it took between refills of DEF, the engine always ran pretty clean. There was never any smoke on acceleration, never any soot whatsoever inside the tailpipe, no stink, no smell, no coal rolling. The engine has been remapped to meet regulations, which nerfed performance, and then re-mapped again subsequently which brought it about 90% of the way back. And I cannot detect any difference in fuel economy (I have tracked since new) nor in visible soot / emissions. It's as clean as it ever has been. I don't even clean the tailpipe, it's always been super clean.
Anyway, sorry about the anecdote.