Some Time ago I wrote a piece on All Old Guys Should Cycle. I explained that I set a minimum goal of 1000 km per summer.
Well, last night I rolled my odometer over the 9999 km mark and of course it resets to 0. In fact I have rolled the 9999 two and a half times. Once my battery died and I had to start over again. However, the odometer helps to keep up my will to continue cycling as it sets a very definite goal and if I set something which I can achieve it works.
I have a Cateye speedometer which is quite accurate. It records time, trip distance, accumulated distance as well as speed. There is also a clock for regular time. Fancier speedometers are available which record heart rate if you really want to go for fitness.