That's a far deeper question than you realize I think.
It will basically come down to how the game is programed. If they perfectly render motion blur, you could get away with as low as 18 fps (pre hd movies were here). Since that's simply not possible yet, games tend to shoot for the 60fps mark, so when your hardware can't keep up, you can't tell.
It has been shown that the eye can recognize the existence of 1/300th of a second blink of light, so if they have absolutely no motion blur programmed in (highly unlikely) it would take up to 300 fps to remove the flicker.
So I guess yeah I consider 60 fps to be the relevance cap, but again it all comes down to the motion blur being able to trick your eye into not realizing those are still frames going fast. I've not played Iracing, so you'll have to make your own judgement on the quality of their motion blur.
This can explain it all much better than I.