Electronics, Lighting, & Gadgets - installing a used small speedometer/cyclometer?

i have an older small cyclometer that is on my brothers bike that he said i could have. i am not familiar with them at all, but i see where you put the sensor on the wheel spoke and wire the other part on the fork to pass by it with each tire revolution, but my question, does this thing need to be calibrated in some way? the bike it is coming off is a Gary fisher Mamba with 26" rims and mountain bike tires....mine is a Gary Fisher Marlin with 26" rims and road tires.....so the circumfrence should be the same on each......any tips on this install?

What's the make and model of the comp? Especially calibrating it requires manufacturer's instructions (or advice from someone who has a similar comp). If you don't have the manual, you could try to search for it from the manufacturer's web site.

Generally spekaking you should install magnet on the spoke and sensor on the fork in a way that makes the magnet pass the sensor fairly closely. As an example, for Sigma computers this means max. 5mm between the magnet and the sensor, as per manufacturer's instrucions.

I'd say it's a good idea to calibrate it if you move it from one bike to another, even though both are 26". Tyres can make a lot of difference (relatively speaking). I use the rollout method myself: put a bit of water anywhere on front tyre and ride straight ahead on a dry, hard surface. The wet spot on the tyre leaves marks on the ground. Measure over a couple of those and divide to get the length of one full revolution.

How you enter this info into the comp is dependent on make and model of the comp.

--J