@NicoNivoSo to bump this up, and shed more light on my findings. Today I've been on GP mode on Xbox, around Hungary as Mick Schumacher. Qualified 2 tenths ahead of K. Mag. And for the first 3/4 laps of each stint could at least match him, maybe slightly faster. But after that he passes and pulls away and I'm just trying to keep the baying pack behind me, as if tyre wear hasn't had an effect on them at all. by the end of an 18 lap race, K.mag finished 20 seconds in front.
So It's either they aren't affected by tyre wear or the player car doesn't lose any weight while burning of fuel, in my humble opinion.
And before anyone replies saying it's not, please do this test for me. Should only take 30 mins.
Go to GP, set the race to Hungary, pick any team you want but try and get the level, so you qualify around your TM in short qualifying. 25% race distance. Use the default set-ups (which I'm still assuming the AI uses) in a clear session. And tell me what you think.