Wrong tube size.

Yes the size is wrong. Too small.

Use a Conti "Tour 28 Slim" for 28 and 32mm tyres.

Am I right to say there will be a significant air leak if the tube size used is too small?

How bout, is it possible to use a oversize tube for thinner tire? for example fixing a 28C to a 25C?

Macross said:

Am I right to say there will be a significant air leak if the tube size used is too small?

How bout, is it possible to use a oversize tube for thinner tire? for example fixing a 28C to a 25C?

There should not be an air leak from an undersized tube. The problem is that the tube will not fill the tire and the tire may not seat properly on the rim.

If you can get the 28 tube to fit easily in the 25 tire, it should work fine. There seems to be quite a bit of variation between manufacturers in the actual size of the tube (or tire) for any given size. Some 25 tires are quite narrow and you might not get the 28 tube to fit without twists, puckers, or kinks.

We have a situation of coincidence here.

The first "28" in the size is the wheel diameter in inches, the second 28 is the inflated tyre width in mm.

Have a look at the Conti web page on tubes,
http://www.conti-online.com/generator/www/de/en/continental/bicycle/themes/tires/tubes/tubes_en.html
for the 23/25mm 28 inch tyre, use a "Race 28".

I doubt your second flat was due to using a slightly undersized inner tube. If I had to guess, I'd say the thorn or piece of glass that caused your initial flat is probably still stuck in your tire.

I prefer to use inner tubes that match the tire size. If, for any reason, I can't find the exact size, I will always opt for a tube that is slightly undersized rather than one that's a little too big. Oversized tubes are hard to stuff into a narrow road tire and are easily pinched under the tire bead.