Relax Dan! I have just made a new speed sensor disc that bolts onto the gearbox shaft in place of the standard disc. The standard disc, or ours at least, has 8 spokes to trigger the sensor. I just made a new one with 13 spokes which, with the adjustments that Rolf has figured out will put us right in the middle of the metric speed range. It was very simple to make a new disc out of 3mm scrap and drill 13 holes. Took a couple of hours. Cheers!