Date registered: Mar 2009
Vehicle: 1987 300D Turbo (RIP), 1995 E420, 1993 400E
Location: Madison, WI
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Hmmm... I haven't measured my center caps off my stock rims, but I'm pretty sure that they're smaller than 3 inches. Smaller than 2.5 inches actually. Do you still have your stock rims? If the cap from your stock rim doesn't fit, than you may have a problem. I know that the center hole on just about every MB wheel is standard. So much so, the ones on my car are actually off of an old 280SL. They're the enameled ones with the white star and blue circle, etc.
The decals, though not ideal, may be your only choice. Unless, you buy a set of stock caps, and modify them so that you can epoxy them to the current caps, or some other solution along those lines.
I knew a guy who had some wacky wheels on his 300CE. He drilled out the center of the caps that came with the rims so that he could snap in and epoxy the MB caps into the aftermarket rims. It actually looked really good with a sort of tiered chrom ring affect.
Best of luck,