tire sizes do not match correctly

Question

I have a tire size of 40-622 and 700X38:

But when I look at the table I see that it does not match:
40-622  28 x 1-5/8 x 1-3/4  700 x 40C   28 x 1½ x 1,75  1995    Find tires
38-622  28 x 1-5/8 x 1½     700 x 38C   28 x 1½ x 1,75  1995    Find tires

You see how on the first line we got 40-622 (in the first column) and in the second line we got 700 x 38C (in the third column)? So what's the actual size of my tire?

Adam Rice · Accepted Answer

Tire sizing is a mess. Read it and weep.
"700" or more properly "700C" is derived from an old French tire-sizing system; the number indicates the outside diameter of the tire in that system (700 mm), and the "C" indicates the width of a tire under that system (meaning "wide,"). "622" represents the bead seat diameter (or, approximately, the inside diameter of the tire) of a 700C tire, under the modern ETRTO sizing system. These days we use "700C" as the name for a size with a tenuous relationship to its original measurements (curiously, 700C has mostly survived as the name of the size for skinny road tires); "622" is more precise.
You'll see that these tires are also called 28" on that table, which is extra-fun because they're slightly smaller than 27" tires (which have become very uncommon), but have exactly the same bead-seat diameter as tires labeled 29" (used on mountain bikes).
Why the widths (38/40) aren't the same is less clear to me. Maybe the company has their own system for measuring widths that is slightly different from the ETRTO system.

Criggie · Answer

Difference between 38 and 40 is 2mm, so there's probably more size variation than that between different brands.   Either is fine, depends which one you can find to buy locally.
In theory wider tyres are squashier, for more comfortable ride, as long as they fit inside your frame.
The slightly smaller tyres would be slightly faster, and being smaller would definitely fit in the frame.

tire sizes do not match correctly

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