Will the S&S Speedometer calibrator work for me?
The downside to making changes in gearing or wheel sizes is that it throws off the speedometer reading. If you've got a pre '96 bike, or custom bike using a mechanical speedometer, you have to change either transmission speedometer drive gear, or front speedometer drive unit, or sometimes even have to change out the speedometer to a different ratio to correct the speedometer reading. Even then, readings may still be only within a couple of mph of being accurate. That's where bikes equipped with electronic speedometers are really cool. Either stock or custom bikes can be set up with a '96 and up style electronic speedometer, and in a way that allows changing the speedometer reading without the trouble of swapping out parts to get a correct reading. The 1996-up OEM electronic speedometer itself isn't adjustable. To realize the full potential of the electronic speedometer, S&S designed a nifty little unit that plugs directly into the existing wiring and requires no tools or test equipment to use. It is makes accurately calibrating 1996-up OEM electronic speedometers a snap! It fits all Big Twin and Sportster models originally equipped with electronic speedometers. It's also great for custom builders using a '96 and up OEM style speedometer and transmission .The calibrator allows builders freedom to create without concern for speedometer accuracy, because the unit can be adjusted so that the speedometer reads accurately with whatever combination of gearing and tire sizes has been chosen. In most cases, it can be set so that the speedometer is accurate to within .5 percent of actual speed. What does .5percent mean in mph? For simplicity sake, let?s use 100mph as an example. 1 percent of a 100 mph is 1 mph, and half of that one percent (.5) is one half mph. So, recalibrating your speedometer to read within one half mph of actual speed is possible. It is important to note, though, that the calibrator is designed to work with OEM components, and may not be compatible with aftermarket speedometers or transmissions. One notable example is that the calibrator will not work with Rev-Tech transmissions, unless you piggy back two units together. A fairly recent development is that Harley Davidson changed the voltage used by the speedometer system from 12 volts to 5 volts. Sharp-eyed customers have noticed this, and our Sales and Tech departments have started getting calls asking if the S&S calibrator will work with 2004 & 2005 model Harleys. The answer is yes. S&S Test and Development department has verified that the calibrator will function at five volts and the unit has been tested on 2004 & 2005 models with good results.

Source: S&S Website