Tire sizing.

Now this may be a stupid question but here goes.

I am setting up my cycling computer with the new tires I just bought.

The tire size is 26 x 1.35.

So I am wanting to put the tire size in to the computer but it does not have it listed in that way.

The table of sizes goes

26x7/8
26x1(59)
26x1(65)
26x1.25
26x1-1/8 <this one?
26x1-3/8 <this one?
26x1-1/2 <or this one?
26x1.40
26x1.50
26x1.75 and so on.

26X1-3/8 is actually 26X1.375. This is the closest to your tire size of all of the options that you have listed. I have never encountered a computer that gives these options. All of the ones that I have ever set up wanted the wheel circumference in millimeters. The most accurate way of finding this is by doing what is called a Roll Out. You mark the pavement and position the wheel centered on the mark with the valve stem at the bottom of the wheel, pointing straight up. Roll the wheel one complete revolution, using the valve stem as a reference to determine the end of the revolution, and mark the pavement at this point, centered on the center of the wheel. Measure the distance between the marks, convert it to millimeters if required, and enter it in your computer. The reason for doing a roll out is that no two tires of the same listed size are going to actually be the same size, especially if they come from two different manufacturers.

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I am afraid I have to disagree on the 26X1-3/8.

The 26X1-3/8 rim is 590mm in diameter. Typical MTB/hybrid/cruiser 26" rims are 559mm in diameter. The tires won't interchange. I know 1-3/8 mathematically equals 1.375 but a rim called "26X1-3/8" is a different diameter than common 26" rims. It just is.

There are, I think, eight different rim diameters in somewhat common use which are called "26 by something". You gotta think of it as a name rather than a dimension. Most decimal rims i.e. 26X1.95 typically are the mtb/cruser/hybrid size, 559mm.

If they use the table of tire sizes to correlate to a number such as "2068" then I think that is the development in mm, i.e. how far the tire will roll if it goes around once. You can measure that yourself, but make sure you do it while sitting on the bike just as you would while riding, and load any panniers with whatever you plan to carry. You may need a friend to help steady the bike & measure. The sag of the tires under load is another reason to do a rollout.

The rollout as described by Kdelong is very simple and accurate. Pumping the tire to the pressure you will be using and doing it with your weight on the bike will replicate actual conditions and then your computer will then be as accurate as possible.

On the two bikes I did the rollout on I have found a difference of about 5mm per revolution compared to the tire chart.

+1 just roll it out... that is the easiest, most accurate way of doing it... put a piece of tape or something on your tire so you know where you started an will know when you've done one rev and then roll it out with the tires pumped as usual and your weight on the bike...