Speedometer calibration

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.
This seems odd to me. Where is the speedometer taking its information, which is unaffected by the gear ratios?
Can someone give me a definitive answer? Also, I would like to know where the speedometer is getting its input if the answer is "no recalibration required'.

Thanks again to all!