How about a gear switch indicator? Showing you when it's most efficent to switch. You would need to know the position of the switches. Of course the main work is creating the table. Additionaly you could indicate how you should switch since often the best combination of front and rear gear isn't realy easy to know.
With an accelerometer you could even detect if you are currently accelerating or just maintaining speed, that way you could optimize the switch indicator for different situations.
Cheers
Björn