I found that if I had my pedometer in the wrong place on my belt I would get a bad reading (much further than I had actually walked). Maybe it was a bit of a `bounce' effect causing it to over measure. The best place I found was on my belt just about half way between my hip bone and belly button.
I had a hard time working out stride length, so I measured a distance I was walking (by driving it in my car

) and then adjusted my pedometer until it gave the same reading. I later confirmed the distance with a GPS and it was very close.