I started with a speedometer in a can which was mechanically activated by a flange stuck between the spokes. Not accurate in the least, but it was fun. Then moved up to very crude magnetic speedos with a wire running down the fork, which gave speed, ave speed and distance and time. Outrageous amount of info! Now I have a Garmin with satellite GPS that is so sophisticated, it slices and presents 25 pieces of data in any format I want, has rear radar, alerts me to messages, weather conditions and provides turn by turn navigation. Can’t imagine what they will be like in another 10 years.

Edit: My iPhone is always in my jersey pocket in case of emergency (mine or others) plus it relays messages, incoming calls to Garmin.