Dirt on tires not showing

The dirt that appears on the tires after going through some mud isn't showing anymore.
I've reinstalled the game several times hoping to fix it, but that didn't work. Does anyone have a clue as to what I am missing? Is there a "config" file that needs tweaking? I have installed a couple of maps other than the stock ones. But even reinstalling the game to stock doesn't seem to fix it either.

Thanks!

Make sure you have the highest graphics and simulation settings set because on the lowest settings mud on wheels is turned off.

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