Help Measuring Bicycle Rims

[cpcnw]

Finally caved into acquiring another bike when I saw a pretty shabby Raleigh mixte dumped by side of the road. Frame is at the bead blasters as we speak. I've salvaged as much as I can but the back wheel is shot. It seems to measure all the way across at widest 645mm

According to Sheldon's blog https://www.sheldonbrown.com/rim-sizing.html this is a 28 x 1 1/2" however when I measure the 'outer rim width' with my cheapo electronic callipers I get 24.9mm

So, what the hell is this? Was it some hybrid Raleigh only thing? I am keeping the front as that's not so bad so I need to replace the above. Recommendations would be great. And also suitable road tyres?

Thanks in advance.

Edit: since looking back at that link I think I can answer my own question - it's the row below i.e. 27 x Anything

[Pickled Onion]

Firstly, you need to know the bead seat diameter. At the moment you're just guessing this is 10 or 15 mm less than the outer diameter. It could be either (or something else entirely).

Secondly, the 1½ in "28 x 1½" is not the rim width, it's the tyre width. So that is not implausible.

Lastly, it probably doesn't matter. Unless you're restoring this as a museum piece, throw both the wheels in the bin and build some in a standard size. You'll have a hell of a job getting tyres for them and the stopping power will be shocking. You could use the existing hubs if they're any good, and you never know, the existing brakes may well reach.