This does seem to be a bit of a no brainer, but yes you need a speedo for your bike. There are three factors you want to record when touring. The first is obviously the speed you are doing.
Speed
Once you develop a riding style you will benchmark your flat no wind good road speed, mine is about 20 to 25 kph. With this benchmark you can monitor your speed and see the effects of wind, climb, or tiredness, on your cycling. Sometimes you think you are travelling slower, or faster only to be surprised when you see your actual speed on the speedo.
I also use it as a rough guide to my expected time of arrival. It allows you to better judge how long you have to cycle and what breaks you can take to reach your destination within your deadline time. Mental gymnastics like these occupy your mind when cycling as well :-)
Trip distance.
Necessary to know for the "Thank God there is only another 10km to go" type of thinking. Also to record the kudos of breaking a personal best.
When we started touring we would have a pause break every 10kms and a get off break every 20kms. As our fitness increased these breaks naturally dissolved away, but we still would have a pause every 20km or so. Eventually we reached the level of doing 20kms before breakfast, and would often have have 50 to 60kms done by lunch.
A daily record of your distance is a wonderful keepsake for how much work you have done. I still get excited at breaking the 100km barrier on a day, especially as some of those times involve big climbs and hard work.
Total distance (odometer)
This is a must have ability, one of the pleasures of cycling is seeing those kms flow under your wheels. It also allows you to better judge how long your trip will take, and if you have to be at a certain place within a time period.
A Watch
It beats having to cycle with a watch on your arm, one less thing to have to worry about.
What you don't need
An "average speed". This does nothing but promote the "I must do better next time" mentality, the antithesis of cycling. Beware of falling into the trap of trying to beat your previous records.
A heart monitor or cadence monitor, you are in the saddle for a long time, you will work out your cycling style as you go along.
An expensive bling system. Get a very simple one and forget the "steal me" sign that it gives to other people. If you lose a $20 system, its not much, lose your $200 system and you have lost a lot.
Bike Speedo Summary
- Get a simple system for your bike.
- make sure its solid enough and weather proof, such as Cateye brand
- Don't use anything that you might regret getting stolen
- Don't slide into the "I must beat the speedo" mentality, use it just as a tool
- Cycle touring without a speedo can really hamper your trip and decrease your enjoyment.