tyre & tube size confusion

Hi,
can anyone enlighten me on the flexibility in matching tube sizes to wheels/tyres
I have 28 wheels (hybrid bike) and current tyre is 40-622 (700 X 38C - 28 X 1 5/8 X 1 1/2) .... incidently, anyone explain what each component of the code reflects? ..boy, I thought car tyres were complex
Anyway, my pressing question is what range of tubes will fit ..... the current tube is a 28 X 1 3/8 X 1 5/8 which doesnt exactly match the tyre but works fine ...so I'm figuring there's a bit of leeway here with tubes???....
the reason for my question is that I'm finding out that the 28 size is pretty much obsolete in Australia and I can't find 28 tubes anymore ...sooooo, could I use 27 tubes (will they stretch and is there any implications on max inflation ... Ok you can tell physics isn't one of my pet topics) ...
Anyone advise?
Ta muchly

The 622 refers to the actual size of the rim in mm. 700c is a nominal outside tire diameter, and is rounded up from actual. If you use tubes that are bigger than appropriate for the tire, they're going to bunch up and you'll risk tearing them during installation. You can probably get away with a slightly smaller tube. Maximum inflation pressure is determined by the tire and the rim. Once the smaller tube inflates to fit the space, it stops getting bigger. There might be a bit higher risk of puncture though, since the rubber of the tube is stretched thinner.

622 is the bead seat diameter (622 mm) - which is the 700C size tyre which is not the same as the old 28 inch tyres of the 70's to 80's which were 700A (642mm bead seat diameter) 28x1-3/8 or 28x1-3/4 or 700B (635mm bead seat diameter) 28x1-5/8 or 28x1-1/2

27 inch tyres have a bead seat diameter of 630mm

from the 622 figure on the tyre, (assuming that the tyres are the correct size for your rims, you would have 700C tyres and rims.

You should be able use tubes of size 700C for hybrid bike tyres, which are much more readily available than 28 inch tubes.

The 28 inch marking would be a loose nominal sizing.

here is a table which might make this clearer. see also Sheldon Brown tyre sizing

mike