Alloy wheel centre caps

I hate to state the obvious, but can you not remove the one off the other size to measure it?

Colin

.....and take it with you when you visit factors. 

Both have been pinched 

The facts given earlier would have saved two of us from making pointless suggestions. Never mind. Good hunting.👍🏻

If you have a pair of inside calipers or vernier gauge/calipers you could measure the internal bore of the wheel. I'm guessing you do not, so cut a piece of stiff card from the likes of a food packet so it will just slide in across the diameter of the hole touching the opposite sides of the hole. You should now be able to work out the size.

Colin

Just measure the hole with a rule o tape measure

Lost one of mine from a 2015 Coachman. Found a site on the internet that gave the size of the cap for the wheel and then ordered. Although only plain bought 4 for less than half the price of buying original which I considered to be very overpriced.