I believe that Chris Juden of the CTC knows everthing there is to know about tyre sizes. He mentions the 650B which is the same as the English 26x11/2 ISO size 38x584. Also the 650c (no English equivalent) ISO size 40x571, but no mention of the 650D. However if it does exist it should be fatter than a 650c but fit on a smaller rim. As I understand it the idea was that as the tyre got fatter and deeper the rim got smaller so that they all had the same outside diameter and circumference. Saves messing about with your trip computer when you fit your loaded touring wheels I suppose but you've got to lower your brake blocks! Incidentally Jock Wadley, editor of Sporting Cyclist and one of the first three Brits to ride P-B-P (1971, I think) had a bike built to take 650B in the days when Northern France was cobbles from end to end. He reckoned it was a waste of time and money and 27x11/4 were just as good. Ray Green, Brighton, England
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