I recently purchased a Shimano BC 1009 cycling computer. I took my bike for a spin after installing the computer and everything worked fine. However, the very next day, I noticed that the computer was not accurately measuring my speed--specifically, the speed that it displayed was much faster than I was actually going.
I have no idea what the problem is. I double checked to ensure that the wheel size was properly calibrated; the magnets are properly positioned and not too far apart; moreover, I can't attribute the inaccurate reading to wear-and-tear because I barely logged more than 20 miles on the computer prior to noticing the problem.
I would appreciate it very much if anyone could provide me with some insight as to how I can go about fixing this problem.
I checked mine with a gps phone and the numbers where the same, allthough the bike computer updates the speed much faster then the gps so you kinda need to try to stay at the same speed for a while to check the numbers. Basically I read somewhere that the magnet needs to be positioned as close to the hub as possible so the speed of the magnet when it gets by the sensor wont be too fast so the sensor wont miss counting any revolutions. Another issue might be that your tires are not the same size anymore as they used to be. My tires are 700x35, but since I use them for some time now there are probably a few mm less now due to wear. I would try repositioning the magnet again, closer to hub and check all the settings once more. Good luck!
