By Stan Purdum
If you ride often, the time will come, because of treadwear, deterioration, embedded sharp objects or a notable worsening of how the bike handles, when you need to replace your bicycle tires. Since not all bicycles have the same size wheels — and there’s a bewildering array of tire sizes available — it’s important to get the right tires for your bike. Most of the time, however, sizing is simple: Just look at the size marked on the sidewall of your existing tires and purchase replacements in the same size.
That said, when buying new tires, it’s a good time to consider whether a different type of tire in the same size, and even whether a wider tire, but in the same diameter as your existing tires, would be beneficial.
This article explains what tire-size numbers mean — in some cases perhaps telling you more than you need to know — but giving a place to start in case you might want to try tires of different widths or styles on your bike.
Understanding Tire-Size Numbers
The first thing to know is that mountain bike tires are usually sized in inches, and road bike tires, except for those on some very old road bikes, are sized metrically.
For example, a common mountain bike tire size is 29 x 2.25, meaning that the outer diameter of the tire is 29 inches and the width of the tire is 2.25 inches. Mountain bikes that are a decade old or older will typically have 26 inch tires, but this size is uncommon for newer bikes. Another mountain bike option is 27.5 inches. 26, 27.5 and 29 are references to the wheel diameter, or how big across the wheel is.
A common road-bike tire size is 700 x 25c, meaning the outer diameter is 700 millimeters and the width is 25 millimeters. The “c” is a historic relic from an old French system that indicated the rim sizes using a, b, c and d. The letter has no use today and can be ignored.
With the advent of gravel riding, a slightly smaller wheel size called 650b has made a comeback, and you’ll find it on quite a few gravel and adventure bikes. The reason bike manufacturers use it is because you can often fit a wider tire inside the same frame, since the wheel size is slightly smaller. So, for example, a gravel rider might be able to fit a 650b tire that is 48mm wide, but only 40mm if he or she used a standard 700c wheel.
To make things more confusing, the 650b wheel size is the same as 27.5 inches. So as long as the wheel doesn’t have such a wide rim that you can’t effectively put a road or gravel style of tire on it, 650b and 27.5 are the same thing. Mountain bikes often use very wide rims, so if the wheel is called 27.5, you’ll want to make sure it will really work with the tires you plan to use.
Some older road bikes, particularly those from the “10-speed” era, have tires sized in inches, with a typical tire being 27 x 1¼. The 700-mm tires common on modern road bikes and 27-inch tires are close enough to the same circumference that inner tubes for one will generally work in the other, but the tires themselves are not interchangeable.
Just to confuse matters, or so it seems, there is a third size-marking system from the International Standardization Organization (ISO) that uses metric measurement for both mountain- and road-bike tires but reverses the order of the information.
For example, using the ISO system, a road-bike tire that would otherwise be marked as 700 x 28c, would be designated as 28 x 622, with 28, the width in millimeters, given first, and the 622 the inner diameter of the tire (measured across the tire from bead to bead) given second.
A mountain-bike that would otherwise be marked 27.5 x 2.25 inches would be designated in ISO as 55-584 millimeters. Because the ISO system encompasses all tire sizes, regardless of whether they were originally developed in inches or millimeters, it has the potential to become the worldwide standard tire-size system.
So far, however, ISO numbers aren’t common on tires made for sale in the United States, but are starting to show up, sometimes in addition to the more common inch or millimeter size numbers. But unless the ISO number is the only one on a tire, you don’t need to bother with it. Should you need to convert an ISO number to U.S. sizing, here is a conversion chart (on the chart the ISO numbers are noted as ETRTO for the European Tyre and Rim Technical Organization, which developed the ISO system).
One more word about tire sizes: Regardless of what size-marking system is used, some tires marked as identical sizes will fit more tightly or more loosely on your specific rim. That’s because tires from one manufacturer may be a fraction of a millimeter different in size from those from another manufacturer. And the same is true regarding rims from various manufacturers.
These slight differences can make tires harder or easier to mount on your rims. There’s no chart showing which tire-and-rim combinations fit looser or tighter, so you can only discover these by experience and advice from others cyclists. But with enough effort, you should be able to mount any tire on a wheel designated for that size.
The tires that came on a used mountain bike I recently acquired work fine but are so tight to remove and remount that I carry, on that bike, a special tire lever made for tight-fitting tires.
Why You Might Want to Upgrade to Different Tires
Some upgrades are simply for quality. On my road bike, I’ve run a lot of tires of different brands and different models — to say nothing of different prices. Eventually I settled on one brand and model that gives good performance and doesn’t flat frequently. But all those tires were of the same size. As my tires wore out, I continued to buy new ones in the same size that came on the bike: 700 x 25c.
But a few years ago, while riding after back surgery, it seemed like I was feeling every bump, so I changed to 700 x 28 tires, which made for a cushier ride. I might have gone even wider, except that there was not enough clearance in my front fork and between my rear stays to do so.
Other reasons you might want to upgrade include getting a tread pattern more suited to the type of surface you usually ride, getting more responsive or lighter-weight tires or ones with more puncture resistance. Some tires are designed for greater speed and others for greater load-bearing, which is important if you are a heavier rider or are doing loaded touring or bikepacking.
There are limits to how much wider you can go without getting wider-rimmed wheels, and as mentioned above, frame and fork clearances can impose limits on tire width. In fact, some carbon road bikes from just a few years ago only have enough room for a 25c tire, because it was much more common for riders to ride narrower 23c tires. The current trends continue to skew wider, and some of the most recent road bikes have room for up to 30c and 32c tires.
It’s also possible to go with too narrow a tire for your rim. In both cases, such mismatches can cause handling problems and possibly accidents. Bikes shops are usually a good source of advice for matching tire widths with wheels.
Tubed or Tubeless Tires?
Everything said so far about tire sizes applies to both tubed and tubeless tires, but it is possible to upgrade from tubed to tubeless. However, unless your wheel rims are already tubeless compatible, you will need to either purchase new wheels, have your present ones rebuilt with tubeless-compatible rims, or use a tubeless conversion kit with your existing wheels.
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Stan Purdum has ridden several long-distance bike trips, including an across-America ride recounted in his book Roll Around Heaven All Day, and a trek on U.S. 62, from Niagara Falls, New York, to El Paso, Texas, the subject of his book Playing in Traffic. Stan, a freelance writer and editor, and Methodist minister, lives in New Jersey. See more at www.StanPurdum.com.