Another dunce question

[paulbuckle]

Thanks bonj. I did look in the manual in the end anyway to get the correct number. Seems odd to have 2 (or more) systems for tyre sizes...

 

[Freewheeler]

Tyre sizing is pretty complicated, Sheldon Brown explains the different systems here. Older systems measure the outside diameter of the tyre as you would expect, but the more critical measurement is the diameter of the bead where it sits on the wheel rim. This is the measurement used in the ISO system (650C, 700C etc).

Oh, and my 26x1 3/8 tyres also have 650x35A embossed on them.

 

[ufkacbln]

The CTC do a similar chart, but has the advantage of "comparing" the different sizes.

Calibrating the computer can be done more accurately with a "roll out".....

This is a more accurate method, and just dropping it as a suggestion will impress!

Prepare your bike as for a ride with tyres at your normal pressure
Get some chalk, and place the bike so the valve of the wheel is at the bottom.
Mark a chalk line at the valve position on the ground
Roll the bike forward so that the valve is at the bottom again.
Mark the position
Now measure between the two lines.

This is an accurate measurement of the tyre / pressure/ wheel combination.

Depending on the computer it can be possible to use it for both - have "bike "1" as the 26" and bike "2" as the 700C