I lost one ( as I was ripping my tire weel out getting hooked on my mud flap)......But I still leave them on...
$10 to replace them here in Vegas !

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Most of the 3s I see off road have the center cap still on. I don't remember ever seeing anybody lose one but I may have just not noticed.

 

Well, you can tell from my siggy that I learned to take them off. The first "real" mud that I hit it pulled one off so I just took them all off. Problem with doing this is that your rims get the evidence of off roading then when you put the center caps back on you can tell that they are much newer looking.