Dolphin can only limit the framerate by limiting the game speed. So if you want to set the NTSC framerate to 12 fps, you can get the same effect by setting the emulation speed to 20%.
Also (and I don't know how many times I've said this already): Cars 2 is not a normal game whatsoever. I don't even think high-end PCs can run it at full speed.
Also (and I don't know how many times I've said this already): Cars 2 is not a normal game whatsoever. I don't even think high-end PCs can run it at full speed.