Do you have one of the fancy AutoBike Voyage bikes? It’s a automatically-shifting bicycle from a company at EvolveTheBike.com. The NuVinci continuously variable transmission automatically and smoothly pops you from one gear-ratio to the next. You don’t have to bother with shifting; you just have to pedal.
No surprise, the bike has a small computer built in to pick the right gear ratio you should be in. And that computer has bluetooth! (Specifically, it does the Bluetooth SPP (RFCOMM); the computer then sends packets of bicycle status data.)
The Speedometer for AutoBike Voyage is a program I wrote to be my new speedometer. It shows you your current speed and cadence, the current gear that you're in, and more.