I use Photoshop, so can motion blur on demand
The dirt was never really meant to be realistic, and obviously it was just stuck on to give a more "rally" feel. I don't like to see clean rally cars
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But motion blurring each and every dirt pixel sounds like a thankless task, especially when optimising it for jpg is just gonna totally screw it around anyway!
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So it's not that effective when it comes to looking like dirt, but it's not too bad when you see it in motion.
Thanks for the suggestion though - I'll look into blurring if I ever try to add dirt in to another skin!