Letters as numbers: modified VS sprint car  

I just got to thinking: we're all familiar with the practice of using a letter (or letter) in place of the usual number on racecars. But I recollect that very few sprint cars used a letter, compared to modifieds. And I'm talking letters only, not a combination of numbers/letters. On sprints it seems X is the most popular letter (Mark Keegan, Mike Herbert, Todd Wise). Let's see how many of each you guys can come up with. Each class.