I use it set at 30 (or 40). It's usefull so you can keep settings higher, you don't have to do your settings based on race starts at all as the hard drops usually only happens when all cars are together. It pretty much only active when it's rainning at the race start, and usually for the first corners, after that things get stable again It will lower settings and sometimes cut objects (like oponent car wheels, but I think this depends on the mod) till your fps goes above what you set. You'll want to turn it off to watch replays tho
I don't think it works very well, you'll have it cutting opponent cars in front of you in worst case, plus I doubt it's smooth because it can't predict the frame dips. My approach is to make sure my graphic settings can handle 60 FPS minimum and then cap FPS to 59 by setting "max framerate" in player.JSON (same capping would obviously work with 120Hz or 144Hz by setting the value to 119 or 143).
My experience is the same as stonec's, though I use different maximum fps setting technique due to running no sync on 75 Hz monitors (prime number not near a multiple of the half-frame rate).
I don't have exact idea how auto fps works. But let me say my experience. I'm using 1080ti and triple 60Hz FHD monitors. In Targa Florio using DX9, initially it wasn't easy to maintain 60fps not only in practice session start but also in the middle of running the course. After setting the auto fps as 60, fps is still dropped down to about 50 at starting time but it keeps mostly 60(rarely 59) in the middle. BTW one noticable thing is easily I see only one car in front of me. The cars from 2nd front of my car is easily disappeared regardless of max visible car setting although in not far distance. This seems the trial of the auto fps feature to keep the defined fps.
Yeah, surely it has some unwanted effects some times, but it helps! By the way, some settings (when you set them lower) have similar effects (LOD-switching).
