Just some hints from Digital Foundry's tech analyst John Linneman :
There are different trade-offs.
Reflections are slightly lower quality on GT but appear to update at full rate. Forza seems to update all reflections at 30fps instead.
This is an unfair comment to make. The video isn’t what it is due to HDR - it required so much effort to make that I couldn’t dig in as deeply.
That said, I disagree with your assessment now after spending the morning comparing the two.
Forza and GT both use 2D trees but the tree textures are more detailed and better lit in GT. Forza just slaps two flat surfaces together but it’s still ultimately 2D in nature.
GT also uses a fully modeled 3D crowd on all tracks aside from the giant stadiums. Forza’s crowd is entirely 2D.
GT has more details than I expected when looking closely.
Again, this first video was limited because of the HDR production time. It was a nightmare.
I'm working on a more detailed video putting Forza 7 and GTS side by side and I think you're going to be genuinely shocked by the results. I honestly expected Forza 7 to crush GT in terms of track side detail but it's absolutely not the case.
I can see why, though. Forza has a lower contrast look even in HDR and its flaws don't stick out as much while racing, which is a very important thing, but if you look closely at individual assets, you can see that Polyphony put in the work. The tracks ARE detailed.
You seem obsessed with the tire wall but I can point to a number of other higher polygon objects in GT as well.
What I've found is that some objects are more detailed in Forza, others more detailed in GT.
Seriously, you haven't seen anything yet. The difference, when you compare side by side, is nothing like you seem to think. GT has a lot more going on than expected.