I'm gonna take a pot shot at the answer to your problem...for which I'll catch all sorts of hell..'cause I'm a 'newbie' and don't know sheep sh-- from apple butter. But let's give it a try......Somewhere on the back of your speedo head there must be 'dip switches' to help you calibrate your speedo. The best place to start is with your tire Mfr. who can tell you how many RPM's your tires need to rotate to make one mile...pretty much the same as if you were going to install a Hobometer. The pickup will sense the number of rotations of the drum (part of the equation) multipied by a constant in the speedo the (circumferance of the total tread of the tire)...and (light bulb) ...MPH!Now, wasn't that simple?I'm confident that the experts will jump in and give you a solid answer...but I'm sort of capricious this evening and having a bit of fun...not at your expense...mine...after all...I'M A BUSNUT!!!!!!!!