My bike's better than me!
Join Date: Feb 2005
Location: Northern Colorado
Bikes: (2) Moots Vamoots, (1) Cannondale T2000 tourer, (1) Diamondback Response Comp mtb
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1) Many tubes carry the presta valve length marked on the box. If they're not, you can literally just measure them;
2) The proper stem length is NOT a function of your tire size, but rather a function of your rim depth.
A 'standard' (is there such a thing with bikes?) Presta stem is about 35mm. For deep-section ("aero") rims, valves are available in lengths up to 60mm. Several intermediate lengths are available. You really need the one that fits your rim, leaving enough valve available for your pump to 'bite.'