Doing the stressing about gx eagle vs shimano 11-46 and front chainring sizes on a 29er thing…. Trying to see what would cover my existing 24/38 and 36 rear cassette

http://www.sheldonbrown.com/gear-calc.html

hard to knock the late Sheldon Brown but this one including cadence works better for me:

http://www.bikecalc.com/speed_at_cadence

My favourite for the obsessives.

http://www.gear-calculator.com/

+1 for http://www.gear-calaculator.com