My road bike has 700x23C marked on its tires.
Last week, I was curious and Googled 700x23C to see what it meant – of course, so I could figure out what my tire diameter and circumference is to put into the bike computer. And thus I stumbled upon the mess that is bike tire size designations!
There seem to be various ways of designating tire width. I think the Wikipedia article here summarises the ISO, Inch markings and French designation very clearly with a nice diagram: http://en.wikipedia.org/wiki/ISO_5775
For my wheel (which is a very common racing wheel size), 700 x 23C is an old French desingation broken down as follows:
This used to refer to the overall inflated tire diameter. However, I think it’s lost its meaning/accuracy over the ages, and 700 now means a rim diameter of 622mm (across the bead set).
The French system can also append a letter code here to designate the width of the tire. The common “700” tire is actually 700C. Much more info here courtesy of the extremely knowledgeable Sheldon Brown: http://sheldonbrown.com/tire-sizing.html
The second number is meant to be the tire width. Since bike tires are nearly circular when pumped up, one can estimate that the “height” of the tire is the same as the width – in my case, 23mm.
I think this last letter refers to the type of rim. C means Crotchet-type, whereas SS means Straight-side and HB means Hooked-bead.
Putting it together
If I assume that my bike tire is circular when pumped up (so the “height” is the same as the “width”), the overall diameter of my bike tire is 622 + 23 + 23 = 668mm. This makes a circumference of 2098.58mm. This sort of matches the range in various bike computer manuals.
Of course, it’s probably easier and more accurate to just marki a spot on the floor and your wheel, make sure your tires are pumped up to the right PSI, sit on your bike and roll it a couple of revolutions, and then measure the distance to work out the circumference 🙂
Oh, one interesting blog I discovered on my travels today – the Lovely Bicycle – great photos, stories and articles on vintage bikes and cycling – http://lovelybike.blogspot.com/
LiveStrong’s article on Bike Tire Sizes – http://www.livestrong.com/article/336406-explain-bike-tire-sizes/
Mysteries of Tire/Tyre Widths – http://www.tricktrack.org/forum/viewtopic.php?f=2&t=5566&start=0