As you can see in my bike photo above,.. that's my bike BTW, looking down on it today as I rode to work this morning,.. anyway as you can see the speedo/ computer is dead. I needed a new battery. So I went to the local bike shop and bought a replacement. Then I had to set the computer. They are all different and I have several different ones. Of course I couldn't find the instructions to the one I was using. You have to set the wheel size and kilometers or miles.
Well there are only two buttons on it, so some combination will let you in to set it. I finally figured out the combination of holding down the buttons and in which menue. Then I did a wheel roll out to set the wheel size. Bla bla bla. If you came here looking for that information, hold a button down and wait until the display flashes. If there's a four digit number in the display, Roll the wheel out one complete turn and measure it in cm. Just put a metric tape on the floor, and start with the stem on 0. A typical mountain bike wheel will be 2075 and a typical road bike wheel will be 2124. That's close enough. Don't get anal about it, but roll the wheel out if you must.
I'm not too hung up about recording miles or anything. I've ridden enough to know pretty much how far I've gone each summer, but if you tour, race or distance ride, a working computer/ speedo is really valuable to measure your effort during the day. Other than that my advice to people new to cycling is don't get to hung up on how many miles you ride. It's not the best reason to get on a bike.