I've had speedometers since i was in grade 10. 2 mechanical and 2 wired sigmas. The first one was poor, but I had it for 6 years on my old 1 speed Rudge. Then I had a good one on my 1974 + 1989 Raleighs, until 2000. It never failed me and I still have it.

I wouldn't want to go far without one. It's nice to know more or less when I will get to somewhere, or when I am going to be tired. haha Especially on long all-day highway rides. How far to lunch time, etc. I write down the mileage and avg. of my highway trips. The last two bikes i wrote down mileages for the year end and most of my new parts and tires.

I was in 3 asian cities on cheap 1 speed bikes and I didn't have a clue how far it was to the end of the city or my maps.

Besides this, For all my cars and motorcycle, I have kept track of every tank of gas and oil change. etc. I recorded the mileage, price, cost, date and out of town locations. About 10 trips to 31 states.