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I live in a city with plenty of cycling shops, but they are all out of 27 x 1 1/4 tires. Each shop gives me a different answer about the possibility of using a different size of tire. Can I fit a different size onto an old 27 x 1 1/4 rim?

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    You need a 27-inch tire. The width, within reason, does not matter that much -- you could likely use anything between roughly 1" and 1 3/4". But it's hard to imagine that no shop can get their hands on a 27x1 1/4" tire -- it was at one time the most popular "adult" size tire. (Unlike 26" tires, all 27" tires (except for one or two very weird cases) have the same rim diameter, and are interchangeable provided the width is not too far off.) – Daniel R Hicks Sep 19 '15 at 17:26
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    I doubt its a case of can't get their hands on it, but moreso a case of they need to order it. If you have a bike shop that does used bikes or sees a lot of older bikes, thats where I'd check for having it on hand. A shop that only has the newest latest greatest stuff might not have it on hand, but they should be able to order it. – Batman Sep 19 '15 at 20:42
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You need 27 x (some number of inches given as a fraction). So 27x 1 1/8 would fit, 27 x 1 1/4 would fit, etc. Other sizes won't fit. As dlu's answer points out, an easy way to get this right is to ask for ISO 630 tires.

That being said, 27 x 1 1/4 tires are still made in plenty by brands like Continental, Bontrager, etc. so your bike shop should easily be able to order one for you in a few days, or you can buy them online.

See Sheldon Brown's page for details on how tire sizing for bicycles is a mess.

"Brown's Law Of Tire Sizing:

If two tires are marked with sizes that are mathematically equal, but one is expressed as a decimal and the other as a fraction, these two tires will not be interchangeable."

Sheldon has another interesting note: Very old (60's) 27" rims had straight sides versus modern hooked sides, so they can't run as high pressure. However, he says if you get a tire which says hook edge rims only, you can put it on an older straight rim, just not at as high of a pressure.

Finally, tubes for 27 x 1 1/4 tires are a different matter. You can use 700c tubes in 27 x 1 1/4 tires (The right size should be 700 x 28-32 ish -- the boxes will be marked accordingly depending on the brand).

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    Thanks. I am reading Sheldon's article right now, which is fun. I wish I had the chance to meet that man when he was alive. As for ordering the tires, I will do so, but you know how hard it is to wait when you've been working on something and want to ride it RIGHT NOW. Sigh. – Eric Coates Sep 19 '15 at 16:12
  • You might ask some of the local shops if they have some sitting in the trash that would work until they get some in… – dlu Sep 19 '15 at 16:25
  • I suppose you could look at your brakes and see if you could get about 4 mm more reach, and then go looking on craigslist for a set of inexpensive 700c wheels… – dlu Sep 19 '15 at 16:30
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    That is an option, but honestly, its not that hard to get 27 x 1 1/4 tires these days with a little foresight that i think the cost of conversion is not worth it. – Batman Sep 19 '15 at 16:36
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    Agreed. Just trying to think about what could be done TODAY. And if the bike will accommodate the change without a cascade of other changes it might be nice to have the additional tire options that come with 700c wheels. – dlu Sep 19 '15 at 16:38
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The most reliable way to compare tire sizes is to use the ISO / ERTRO sizing. Most tires will be marked with it. Look for a number like this "32-630" (the ISO equivalent of 27 x 1 1/4). As long as you the second number, the bead seat diameter in millimeters, is 630 the tire will fit on the rim. The first number, 32 in this case, is the width of the tire, also in millimeters. That number can vary a bit without problem. If you go much wider you may have issues with clearance at the chain stays or perhaps the fork crown. Within reason, narrower won't be a problem.

27 x 1 1/4 seems to be the most common 27 inch size still made, but there are some others. Take a look at the 27 inch listings at Bike Tires Direct for example.

  • +1 for ISO / ERTRO. 27 x 1 1/4 is by far the most common, but 1 1/8 is easy to find as well. – Batman Sep 19 '15 at 16:39

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