If it's a wheel sensor there are other possibly critical factors that influence the speedo. I put a new sensor in but that made little difference. Positioning, rather than gap is vital. On mine the hole in the brake caliper meant that the sensor was only half-covering the serrated wheel. Created a bracket out of sturdy stainless sheet that lined the sensor up correctly, and gapped it to 1mm. Also did the earth mod. All this helped and meant that the speedo still worked up to about 100 (instead of 60 or so). Then took it all apart again, repositioned and gapped it, but also properly cleaned the serrated wheel, brushed off all corrosion and coated the tops with a bit of POR15. Still get problems, but well into three figures. Turning the wheel to see if the sensor lights is fine to see if it works at all, but it's the capacity to operate at speed (under load) which is the issue. As Geoff says 'it's a cheap pice of s**t' and I'd guess that some work OK, others work for a while, and some hardly at all. Buy ten, maybe three or four will work forever, the rest will start to show signs of failure at some point. Neil