In the game you can to game settings and turn unrealistic tire sizes on. Does this count as modifying the game.

i would say yes it counts as modifying since it takes away from the realism of the game so thats a very good question and im glad you brought it up

So runs with unrealistic wheel sizes would be invalid

that would indeed be the case as long as you havnt turned on unrealistic tire sizes then the run is valid even if your tires look like a steamroller