Leave it up to man to really confuse things. In the early days of bicycling tire sizes was fairly simple and easy to understand. Have you taken a look at it nowadays? It will give you a headache, I tell ya. I am not going to try to explain it as I don’t understand much about it myself or do I care to try to. What I will do here is provide a link to a webpage where the late Sheldon Brown explains this complicated mess.
http://sheldonbrown.com/tire-sizing.html
Along with all the different tire sizes comes the matter of which tires safely fit which rim … which is equally a mess. Forget correct math and common sense and logic. Originally a 26 inch tire measured pretty close to 26 inches on the outside. Of course, that was back when tires were balloon tires. Nowadays it is all different. Oh, there are still 26 inch balloon tires around, but there are so many other sizes as well.
Basically as I understand it tires are measured and designated by either inches or metric. The first number is the outside measurement even though it may be much less in reality. I told you it is confusing and a mess. Let’s use 26 x 1.75 as an example. As stated, 26 is the outside height of the tire. Again, it may be in inches or it may be metric. Most of us are familiar with the Catrike 700 model trike. It was named 700 because the rear wheel and tire are 700 mm. 700 mm is taller than a 26 inch tire. 700 mm is 27.5591 inches. 26 inches is 650.4 mm. Got it? A 700 tire is much narrower than a 26 inch tire. That brings me to the next part of this identification and designation process.
A second number or letter code would indicate the width of the tire. (26 x 1.75, 27 x 1 1/4…650B, 700C…). To add to the confusion we have fractions, decimals and letter designation. It is enough to make your head spin and give one a headache. And to increase the confusion even more 1.75 is not the same as 1 3/4 in tire sizes. Mathematically they are the same, but that is where it ends. These two tires are not interchangeable. So we need to be careful and know what we are doing when it comes to buying tires and installing them on a rim. The width of a tire is very important and critical when it comes to fitting a rim.
I suggest to others that if you don’t know and understand the system in place go to someone who does. Hopefully those working in a bicycle shop can safely and correctly help in this. Just don’t ask me. 🙂 I don’t begin to understand it all. I am satisfied to know what tires my trike takes so that I get the right ones that will fit correctly and are safe to use.
By the way, buying inner tubes to fit correctly can be the same challenge. You don’t want a 20 inch 451 innertube as it will be too big in diameter for a 20 inch 406 rim. Always be sure you are getting the right inner tube. I have even had sales people in a local bike shop grab the wrong one off of the shelf even after I told them it was for a 406 rim. Some of them need to be educated as well. I told you it was a real confusing mess!
Well, do your best to …
KEEP ON TRIKIN’
Tadpole Rider 
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