I've always though that length should really be dependant on you pedalling style. If your a spinner, short cranks. A masher, longer ones. I have an 84cm inseam and run 172.5. I've been riding a 175 crank for the last 3 days (TT bike) and I think I may go with 175 cranks on my new race bike next season. I really like that extra leverage and I am built suffienctly large enough to take adavantage of a mashing style.