Tyre sizes

i did search but nothing came up...

this may sound like a stupid question but, ignoring the frame clearance issue etc...

will any tyre theoretically fit on any wheel (assuming they are the same diameter).

e.g. would a schwalbe marathon 26 x 1.5 fit the standard trek 26's that my shitty off the peg mtb came with? or are the rims to wide for the 1.5'ness of the marathons?

did that make any sense?

sorry, should have posted that in M&F. please feel free to move it.

yes they probably will.

rims have ratings for tyre widths, they are usually on the label, or on the manufacturers website, failing that just measure the things

a standard mtb rim should be ok up for a range from 1.25 to 2.0, as far as I know.

cool, cheers.
it was one of them where i was a bit ashamed to ask...

See Sheldon for a good rule of thumb as to what width of tyre will fit on what width of rim:
http://www.sheldonbrown.com/tire-sizing.html#width

I've got some 29er rims that measure 25mm internally (they're pretty chunky).

What's likely to happen if I fit 40c Schwalbe Marathons?

Sheldon's old chart says it's not ideal. The next size up being 47c which seems a bit monster for the road.