2.5mm is bugger all, and hardly worth it.

These crank length sites are very interersting, but they are not the be-all and and-all of the discussion, and they pretty much say that themselves. Lennard Zinn is probably the most adamant that his proportional ideas are correct for everyone, but the other most often quoted sites (http://www.cranklength.info/ and http://www.nettally.com/palmk/crankset.html) present their ideas as just theories, although they do back their stuff up with biomechanical research.

this is a quote from the Kirby Palm site:

"While I have the utmost confidence in my crank length formula, I do not claim any special expertise in the general setup of a bicycle for a rider. In fact, I had been postponing creating this page waiting for someone who knows better to set one up."

So, before we start quoting these guys as providing us with any hard and fast "rules", just remember that no REAL HARD evidence has yet surfaced that suggests these formulas are anything more than very good ideas backed up by some reasonable data.

As I said on the other thread, this link is probably the most balanced that I've found:

http://www.bsn.com/Cycling/cranks.html