My girlfriends old bike is getting a bit the worse for wear. It’s a motobecane she bought over from France.
The tyres are 650 x 35b – or 26 x 1 1/2. Lots of tyres on the web at 26x 1 1/2 but doesnt seem that simple.
One tyre I enquired about from a web shop, and got this reply “The 26 x 1-1/2 size listed is equivalent to a 650B x 42. This is the only tyre they do in this range with an ETRTO dia of 584mm (650B)”
On a tyre conversion site http://www.harwoodcycles.com/tyre_size.htm
I found the ERTO/ISO is 37-584… which makes me none the wiser.
So can I just bung a 26×1 1/2 on there. The mudgaurds are pretty close to the tyres already.

The 584mm is the important bit. It is the diameter of the tyre bead.

What it tells you is that no normal 26″ tyre will fit.

It is a trendy size for mountain bikes, or an ancient city bike size.

You can get trendy 650b mountain bike tyres that cost a fortune, or dirt cheap old fashioned tyres from some old shop that carries parts for granny’s ancient roadster (what you probably need).

Like this:
http://www.acycles.co.uk/michelin-world-tour-rigid-tyre-650x35b-black-brown-sidewalls-326.html

Beaten to it, but if in doubt, Sheldon to the rescue:
http://sheldonbrown.com/tire-sizing.html

Brilliant thanks. Yeah, it’s definitely more Grannie than trendy.