Bingo. The rule set I drafted had a maximum tire size for every weight range in 100lbs increments. To make it so cool cars wouldn't be able to play, we made the exception that for every 10mm of aspect you were above your limit, your run would be penalized .1 sec.
So as an example, a 3450lbs car would only be allowed a 275 tire on all four corners. He's awesome, so he has 295's on all four. His run would be deducted .2 sec total time.