Ryan, you said
But by your logic this is
totally impossible because rFactor used the exact same data, in a file called .hdv, and GTR2 used the exact same data, in a file called .hdc. See? Totally different!
In terms of "the whole engine" - yes obviously, many components stripped and replaced. But it's just forks. SimBin has "Lizard", Shift has "Gecko". The basic physics data other than the new tyre model and collision model are pretty much identical between them, with a grand total of exactly one renamed parameter to do with anything in the chassis, and a new tyre model (not an idle choice of words - same style of brush model that iRacing switched to later).
Interestingly enough, they don't regard GTR2 as "right", and Doug Arnao said as much months after the release. They tried and failed to completely eliminate bugs in the stock ISI tyre model which made grip levels drop too quickly (and you can see this same attempt taken further by looking at what was tried next in GTR Evolution). You can read Ian talking about the same thing again
here. So, you know, just something to bear in mind if you are expecting CARS to somehow emulate your favourite bugs from GTR2. It's probably not in their plans. FYI Shift's tyre adjustment is called "subtle" in that post while the GTR2 version of the same is referred to as "falling off a cliff". Make your own judgement about what that might mean.