Cyclocomputer

Smart phone used as a cyclocomputer to present data
Smart phone used for electronic map navigation
Basic cyclocomputer showing speed and Odometer readings.
Wired reed switch sensor with spoke mounted magnet.

A cyclocomputer, cycle computer, cycling computer or cyclometer is a device mounted on a bicycle that calculates and displays trip information, similar to the instruments in the dashboard of a car. The computer with display, or head unit, usually is attached to the handlebar for easy viewing. Some GPS watches can also be used as display.