Because they do the fuel economy, so they need to classify car size class to compare MPG of various makes/models of the same class.
But their method of doing so is ridiculous. Many of these cars with the same interior volume are all over the map weight and overall size-wise, whether I can squeeze 15 hotdogs or 15 newfound dogs into the interior is somewhat irrelevant if the one that fits 15 hot dogs weighs 5,600lbs vs 2,400lbs for the one with larger interior space, follow? The additional weight of the physically larger vehicle is going to have a much greater impact on fuel economy (and subsequently emissions) than how well that manufacturer has managed their interior space relative to a car half its size.
I don't know what GM classify their Sonic, they're wrong if they don't use EPA classification in comparing their vehicles(Sonic in this case) with others.
GM doesn't classify it, that's the point! Going by the EPA classification, the Sonic is in the same class as a car massively larger than it simply because they share similar interior volumes. Follow?
EPA car size classification is useful or useless is another story, but as it is now it is the only "Official Classification" of vehicle size used in US.
But it isn't another story, the merit of this classification is what is being debated, because of how little sense it makes.
As I said, if we don't use EPA standard size class then what we do use to compare MPG of various vehicles ? Should we use size class of car rental ? which car rental company should we use ? If not car rental and not EPA which do we use ? A new standard ? Who will develop a new standard(Car manufacture ? Which one ?) ? Who will qualify that standard ?
It is a mess, so for now the only size class we can use to discuss(in US) is EPA.
How about logic and common sense as starting points? A car like the Honda Fit or Chevy sonic are comparable in size. The Civic, Corolla, Dart/200, Focus....etc are again, all similar in size. Large cars like the Charger, 300, Taurus, 7-series....etc are... Large cars.
Implicitly binding oneself to an illogical mechanism simply because it is "official" should, at some point, cause one to question the merit of such an approach. My hope here is that you will have that breakthrough moment during this conversation so that it can continue, as MrHorspwer has made some great points that are being dismissed simply due to his opposition to the EPA system, which, as I've noted, is a view I share.
