This was my initial thought - but look!
"There are 4 "650_" sizes in between these two extremes.
650[Letter]
In the French sizing system, tires are designated by a three-digit number, which may be followed by a letter. The number is the nominal outside diameter of the tire the rim was originally designed for. The absence of a letter indicated a narrow tire; "A", "B" and "C" indicated increasingly wider tires."A" was originally a tire about 30 mm wide, so the 650A rim is pretty large, 590 mm. If you add the top and bottom 30 mm tire thickness to 590, you wind up with the 650 mm tire diameter.
The 650C size was originally intended for a quite wide tire, about 40 mm wide. Top and bottom 40 mm tire plus the 571 mm rim size again bring you to a 650 mm outside diameter, even though the rim was smaller.
With time, however evolutionary processes have led to different widths of tires being applied to the rim, so the nominal 650 mm designation is now more theoretical than practical.
597 mm, 650, is the same as the British 26 x 1 1/4" size used on club bicycles, and was also adopted by Schwinn for use on 3-speeds with a 1 3/8" wide tire. This size is seen less and less, as the bicycles which use it become rare.
590 mm, 650A, also called 26 x 1 3/8", is the size used on the classic English 3-speed. There's nothing theoretically wrong with this size (other than confusion with the Schwinn size!), but the selection of tires and rims available for it is pretty scanty these days.
571 mm, 650C, was originally a wide, balloon tire size, used on many older Schwinn cruisers. These days, however, it is mainly seen on triathlon bikes and time-trial machines. Available tires and rims are mostly very narrow, intended for competition use.
584 mm, 650B, is the focus of this article. This size, also known as 26 x 1 1/2", is most popular in France, where it was the traditional size for loaded touring bikes and tandems, as well as general utility bikes.
The 650B size was never common in the U.S., and it went into decline even in France with the advent of the mountain bike. However, there is a dedicated group of fans of this wheel size, who have been diligently working to restore it to its former glory.
The situation as far as tire and rim availability has lately taken a turn for the better, and the future looks rosy for 650B...."
I think those marathons are 650B. I have a 2003 650C trek wsd43cm with the aforementioned Continentals (ETRTO 23-571). Think the Schwalbes are ETRTO 37-590s;
"The ISO system uses two numbers; the first is the width of the tire or rim in millimeters (The actual tire width will vary a bit depending on the width of the rim. The rim width is the inner width measured between the flanges as shown in the diagram.)
The second ISO number is the critical one, it is the diameter of the bead seat of the rim, in mm ("B.S.D."). Generally, if this number matches, the tire involved will fit onto the rim; if it doesn't match, the tire won't fit."
So the most badass rough riding tire I can get is then a terry:
http://www.treefortbikes.com/product/33 ... 4AodmTYAJwI may just ask my LBS what it would take to put regular old mtb wheels on my perfectly fitted frame. I hate to buy a new bike when I have a light, comfortable, fun one right here, but the 650C bikes are less and less common every year so this is never going to get simpler... and I want a more sturdy and comfortable ride than 120psi racers.