When I race the XRR and shift through the gears down to 2nd, I get a sort of stutter in the engine. I just wanna know whats that supposed to be, and if it slows me down.

I took a mp3 of what I hear, you may try the set and check if it happens to you too.

AFAIK it's caused by overrevving.

the revs always stay between 6k and 7k. Does it maybe come from my diff settings?

I think you're talking about the huge loss of power once you drop into 1st or 2nd, right? I notice that too, I hate it! I think the term for it is called "turbo lag". it's a bit too extreme in the GTR cars in my opinion, and is exactly the reason I never drive the GTRs with turbo (FXR, XRR). I also notice it in FXO & RB4, but it's not as bad as the GTRs that go from "500 horsepower to 50."

Well it does cut the ignition when it changes

Quote from ACCAkut :

When I race the XRR and shift through the gears down to 2nd, I get a sort of stutter in the engine. I just wanna know whats that supposed to be, and if it slows me down.

I took a mp3 of what I hear, you may try the set and check if it happens to you too.

Check the plugs for water.

I did some testing, switched the 45% lock diff to a locked one, and the stutter disappeared. I thought about it being some sort of "shuffeling" in the diff, cause I have already started steering into the corner when 2nd finally engages, so the diff has to work. I dunno

It is caused by diff settings and was very noticeable before pre-load was introduced.

You can influence this behaviour by tweaking either or both coast locking and pre-load. Although the effect can actually be used to advantage. I generally prefer a more open coast and pre-load setting over a tight coast setting especially in longer races because it protects you from over revving and damaging your engine if you down shift a bit too soon.