Bicycle Mechanics - metric and standard wheels

I told a bike store guy I needed a 27" wheel and took home the one he sold me, but at home I realized that it's a 688mm, not 27". It seems a little off, but technically it should only be off by a 1/10 inch at most. Is that a big difference? Will it still work? Should I even try or should I take the wheel back?

Dude, I think you're in the clear. By my calculatornations, a 27" wheel is 285.8 mm, which is 2.2 mm smaller than your wheel. 1/10" equals 2.54 mm, so you're within tolerances. The wheel will probably shrink a little once you put some weight on it and ride it, which was probably factored into design. shrinkage may be worse in cold weather, however...

I've never seen a tire marked 688mm! The traditional tire sizing was based on the outside diameter. However, this is only a nominal measurement and you will find tires that are oversize and undersize. The important thing to know if a tire and rim are compatible is the bead seat diameter. As a result, all modern tires and rims have two sizes marked on them, the traditional size based on nominal outside diameter and the ETRTO/ISO size based on bead seat diameter. For a 27" tire, the bead seat diameter is 630mm. If there is an xx-630 marking on the tire, then it will fit. BTW, the xx is the tire width, in millimetres.

Refer to http://www.sheldonbrown.com/tire_sizing.html for a good explanation of tire sizing standards.

I told a bike store guy I needed a 27" wheel and took home the one he sold me, but at home I realized that it's a 688mm, not 27". It seems a little off, but technically it should only be off by a 1/10 inch at most. Is that a big difference? Will it still work? Should I even try or should I take the wheel back?
How exactly did you discover that you had a 688 mm rim?

Dude, The wheel will probably shrink a little once you put some weight on it and ride it, which was probably factored into design. shrinkage may be worse in cold weather, however...Dude.....This is just so wrong.

Refer to http://www.sheldonbrown.com/tire_sizing.html for a good explanation of tire sizing standards.Yeah, ya really have to work hard to get a 'wrong' 27' size wheel.

Dude.....This is just so wrong.

So's the idea a 27" rim with a bsd of 630 mm could even possibly have an overall diameter of 688 mm... just putting a little humor back into the rocket science, duder.