Okay so I measure the inner diameter of the center cap that would correspond to the hole
size on my wheel and get 74mm.

The outer lip measures at 85mm .

Now, I see ads for 75mm caps and they state this as diameter ,
so I assume not the inner size, meaning a 75mm must be those smaller
ones for W124 , etc...

I get people talking inches sometimes, like 2 7/8 or 2 15/16 or "about 3".

Well, with a tape measure and an eyeball all three of those are "about" 3.

Hence my desire to put this to rest and get some numbers for different
size wheels, different year cars.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250