I suspect the speedometer will be trying to convert pulses to distance traveled, so you need to find how far the bike goes between each pulse signal. I would think your pick-up will read one pulse per revolution- so you could try figuring out how far the tire moves per revolution of the speedo drive and use that number (it would be in millimeters if you were using a Trailtech unit, for example) as your "wheel diameter" figure. I would first mark a spot on your garage floor with tape and set the rear wheel on it, and mark the tire as a start position. Use a piece of wire or the end of an old speedometer cable to make a flag that can fit into the transmission speedo drive. Roll the bike forward in neutral until the transmission flag has made one revolution. Mark the spot on the floor where the tire has stopped. The distance between the tape marks on the floor will be the "tire diameter" figure you need.