Condensed Answer: In most cases, a 700x23c tube can be used with a 700x25c tire because inner tubes are elastic and the nominal difference between a 23mm and a 25mm tire is only 2mm.
Inner Tubes Expend Significantly
Inner tubes are made of rubber known as butyl. The material is very flexible and can reach incredible volume without tearing.
If you try to maximally inflate an inner tube without placing it within a tire, you will see it multiply in size.
Tire Sizes Are Rarely Accurate
The actual size of bike tires rarely matches the nominal values on the labels. For example, a tire that’s officially 25mm may end up being 23 or 24mm when measured with a digital caliper once pumped to the needed air pressure. Meanwhile, a 23mm tire could end up measuring 23.9mm in reality.
Inner tubes aren’t precise either. A tube labeled as 700×18/23 works fine for the vast majority of 25mm tires.
Note: The width of the rim affects the final width of the tire too. A wider rim results in a wider tire. A narrower rim would make the same tire “pointy” and thus slimmer.
It’s possible to buy an inner tube with one label on the box and another printed on the tube itself. This doesn’t necessarily mean that the tube is defective. Some manufacturers do it to sell stock that’s perfectly usable but improperly classified.
Some People Prefer Smaller Inner Tubes
Some riders deliberately use one size smaller inner tubes. The main motivation behind this approach is the easier tube and tire installation. A larger inner tube takes up more real estate and increases the chance of pinching it with the tire. A slimmer inner tube minimizes that possibility.
Another benefit of relying on smaller tubes is that they take less space when stored in a saddle bag or a tool bottle.
FAQ: If 23mm inner tubes can be used with 25mm tires why do people get flat tires?
If a cyclist installs a 23mm inner tube on a 25mm tire and quickly gets a flat, it would be somewhat logical to conclude that the inner tube is the problem. However, this is unlikely to be the real issue. The new inner tube and the flats are probably not related.
The 23mm inner tube would be the culprit only if it stretches beyond its limits and self-destructs on its own which is highly unlikely to happen given the flexibility of butyl.
In most cases, there will be another explanation for the flat tire(s) such as:
- The inner tube got pinched between the tire and the rim during installation.
- Absent or a faulty rim strip allows the ends of the spokes and/or the edges of the spoke holes on the rim to pierce the inner tube (internal flat).
- A previous source of a puncture (e.g., a small glass) hasn’t been removed from the tire, and the rider is experiencing multiple flats because of it.
- The tires aren’t pumped to the needed air pressure. The insufficient air results in pinch flats.
If all of the above is ruled out, only then one can conclude that the new inner tube has been the source of the new flat.
What Are The Downsides Of Using a 23mm Innertube On a 25mm Tire?
- It’s technically not the right size.
If the tube isn’t classified as compatible with a 25mm tire, then it technically isn’t of the appropriate size. Users who want every part of their bicycle to be as precise as the manual says might prefer to buy a new inner tube or replace the old one if it hasn’t been opened yet.
- Tire patches may have a harder time sticking.
Inner tubes are highly elastic themselves, but repair patches aren’t. Thus, the additional stretching of the inner tube resulting from its smaller size could make it difficult for a patch to stick.
- Thinner Side Walls
The more an inner tube stretches, the thinner its outer layer becomes. This results in reduced protection against flats. That said, the effect is difficult to measure accurately.