Frank Krygowski wrote:

Why not stop at 518.5mm?

He used the tape to measure the outside
diameter of the inside bottom (AKA inner
well) of the rim. but you want the bead seat
diameter instead - the diameter of the
surfaces the tires bead sit on.

The well is deeper (or smaller diameter) than
the bead seat. If it were not, you wouldn't
be able to get the tire mounted.

Yes, what I mean is, don't you get the correct
result anyway by starting from the inner well
and then move up to the closest tire?

Specifically, the issue is 609, 622, 630, and
635 rims.

635 rims are the old-school V-rims with spokes
at a center line. The rear wheel comes with
pre-Torpedo hubs, like the Swedish Novo and
most likely other brands for other countries.
So you can identify the 635s by that.

630 are often thin, 32, as in 32-630 or
27x1-1/4, the old-school road bike standard
(which is great by the way).

Remain 622 (the most-common 28) and the "wierd
Dutch size" 609 which is a 27. (I think you can
use 609 on 622 bikes with no real disadvantage,
at least if you have 609 both rear and front.
635 tho you cannot have on 622 bikes as they
wont fit but hit the fender and/or stays.)

I found it is easier solve the rim size issue
like this. Whenever a tire is removed from
a rim, take the rim to a wall, put it against
it and make a mark with the size on the just
removed tire. Repeat every time there is
a rim/tire that isn't already on the wall.

--
underground experts united
http://user.it.uu.se/~embe8573