Condensed Answer: Technically, a 700×28 inner tube can be used with a 700×25 tire. However, the extra size of the tube makes the installation of the inner tube and respectively the tire a bit inconvenient.
For that reason, some riders will prefer to use an innertube of the appropriate or even smaller size.
Tire Sizes Are Not Always Accurate
The nominal size of а tire does not always correspond to the width of the tire in practice. For example, a set of 25mm tires can end up wider or narrower than 25mm once installed on the rim. This wouldn’t be possible if inner tubes had to be of extremely precise dimensions.
The butyl rubber that most inner tubes are made of allows them to expand to greater proportions and serve a wider range of tire sizes.
The Downsides of Using a 700×28 Inner Tube With a 700×25 Tire
- Reduced real estate
Since the inner tube sits within the tire, a larger inner tube makes it more difficult to hook the tire onto the rim due to the lack of real estate.
The valve area is the most problematic because it’s inflexible. Consequently, a larger than usual inner tube could make it really difficult for the tire bead near the valve to fully connect to the rim.
The outcome is a non-secure tire that can potentially pop off the rim.
Whether this issue will manifest depends on the tire model and the rim’s width. If the tire bead around the valve area fits just fine, then the area isn’t compromised.
- Increased Chance of Flats During Installation
The larger profile increases the chance of pinching the inner tube between the tire bead and the rim. Every time this happens, the user risks a pinch flat.
- Extra Volume + Weight
Wider inner tubes come with a bit of extra weight. The additional grams are, of course, non-noticeable but deserve a mention because cycling is а sport obsessed with lightness.
In addition, 28mm inner tubes have a wider profile and take more space in saddle bags and tool bottles.
The Advantages of Combining 28mm Tubes With 25mm Tires
One can argue that a wider inner tube has the following benefits:
- Thicker Walls
The more an inner tube stretches, the weaker/thinner it becomes. As a result, the inner tube offers less protection against punctures.
Since a wider inner tube doesn’t have to stretch as much as a narrower one to fill the same tire, one can speculate that a 28mm tube on a 25mm tire will be less likely to get a puncture.
That said, the difference in terms of resistance between a standard 25mm inner tube and a 28mm one is extremely small. Thus, if a puncture is going to occur, it will more than likely take place with either inner tube.
- Backup Plan
If you get stuck in the middle of nowhere and have to replace a tube, a slightly larger one will get the job done. After all, a suboptimal tube is still better than nothing.
Summary: What You Need To Know
- A tube designed for a 28mm tire can be successfully used on a 25mm tire, but it’s not guaranteed to offer the best performance due to the extra real estate that the tube needs.
- If the rim is particularly narrow and the tube is on the wider side for a 28mm tire, there’s a great chance that the tire bead around the valve area won’t fully seat on the rim. This could create an unpleasant situation such as the tire getting off the rim.
- A wider inner tube is harder to install and increases the chances of pinching it between the tire and the edge of the rim’s wall. Thus, one is more likely to get a puncture when replacing or patching the tube.
- Unless your only option is to use a 28mm inner tube, it’s more convenient to get a tube of the appropriate size. It will be easier to install and a lot less likely to “spit out” the tire around the valve area.