Originally Posted by Strawhand
I have just changed out the front sprocket on 2012 Nightrod Special (international version) from a 30 tooth to 28 tooth sprocket. I have read some blogs which say that the speedometer does not need to be recalibrated as the input for the speedometer is taken from a point where the gear ratios do not effect the speedometer. I live in Malaysia, so I called Harley in Singapore, and they say that the speedometer does not need recalibration when changing the front sprocket.
Your dealer and the others that think this don't know how the VRSC speedometer works.
A VSS (vehicle speed sensor) reads the 5th gear teeth (there are 26 teeth on this gear) on the output shaft of the transmission producing an AC voltage with a frequency corresponding to the speed of this gear wheel. This AC voltage is read by the ECM (electronic control module) and converted to ground speed using a stored parameter called the PPU (pulses per unit distance). The ECM sends the ground speed to the IM (instrument module) where its displayed on the speedometer.
Any change to the bike that affects the final drive ratio or rolling diameter of the rear tire will change the number of pulses per unit distance seen by the VSS. These include changes to either the front or rear drive sprockets as well as the diameter of the rear tire (tire brand, tire inflation, tire wear, etc.). When you change the front sprocket as you did, you can either alter the VSS pulse stream electronically as mentioned by tunderdemon or you'll need to re-calibrate the ECM PPU parameter. If not the speedometer will show speed as if you had a 30 tooth front sprocket so:
actual speed = indicated speed X 28 ÷ 30
= indicated speed X 0.93333
= indicted speed – 6.6666%.
The dealer is probably not allowed to change the ECM due to legal or Harley imposed restrictions but flash tuners like MasterTune or Dynojet Power Vision allow you to alter the ECM PPU parameter.