I think the dirt on tires feature is perfect, seems to act very much like I would expect a dirty tire to...and with the rally thing, yes rally tires get dirt on them, but it doesnt effect the traction on the dirt, because the traction mostly comes from the knobs of the tires not the friction of the outer surface of the tire, but it would be effect if the groves got filled in...but thats a whole differnt subject.