The iPhone app Gears helps you calculate gear ratios.

Wired magazine recently produced a list of five iPhone applications that you can use with your bike. Most are pretty silly and not practical, but the GPS powered cyclometer Speed could help replace a Garmin with a few more features like an interface with Google Maps or MapMyRide. Also, the Gear Calucator is a cool app for those trying to pick the right gear for their fixed gear or single speed or if you are trying to get just the right gear range for your next epic ride. Read the full article at Wired.