# Why does a decimal sized 26“ tire fit my rims but a fractional sized 26” tire doesn't?

I'm replacing some 26 x 2.10" mountain bike tires that I have on my bike with some thinner 26 x 1 3/8" road bike tires for commuting, however somehow even though they both say that they are 26" the new road bike tires are too big for the rim!

I can take the new tires back, what should I look for when picking out a new set of tires?

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Welcome to the wonderful world of inch tire sizes. Inch tires are not sized by the rim dimension, but the outer diameter, and when you change the width of the tire you change the difference between rim diameter and outer diameter. This covers much of it, but it's pretty confusing at best. – Daniel R Hicks May 20 '12 at 19:35
From what the page linked by DanielRHicks tell, these tires are definitely NOT to be used with MTB rims. – heltonbiker May 20 '12 at 20:00
Thanks Daniel, I actually did find that page but I guess I didn't read it in enough detail and assumed that 26x1 3/8" would be the same as 26x1.375! Anyway now that I know how to actually read the site, I'll hopefully find something good next time I go in. – robbrit May 20 '12 at 20:57

The most obvious thing would be ask for Mountain Bike tires (slick tires in your case) at the shop. Just using the words "mountain bike" is enough for any shop attendant.

Now if you want to take a tire and, just by looking, tell if it fits your rims, you must look for the ETRTO code, which states the inner diameter of the tire bead in milimeters, which is 559 for MTB.

So you have 47-559 (read right now from one of my tires) to be equivalent of 26x47 (47mm wide, or roughly 1.9)

Hope this helps

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559 matters. 47, because it's the width of the tire will vary to get a narrower tire. 559-32 would be something to look for to commute on. – zenbike May 21 '12 at 3:10
@zenbike: yeah, 32mm is my preferred width for asphalt slicks, too, which would be 26 x 1.25 – heltonbiker May 21 '12 at 3:37