Recently went through the process myself. Increased the size of my rear gear, and my speedo and odo were WAY out to lunch. Did some checks vs gps and found I was averaging 21-22% higher on the speedo than gps readings. I'm at 13/48 gearing.
Ordered the SpeedoDRD that motokid provided a link to above, watched a youtube vid on install (don't like reading instructions), and installed in about 5 minutes. Install was removing the seat and left side cover, plugging it in inline, and setting the percentage of correction. The product website has a handy calculator to help you find the correction settings based on tire size and actual gearing vs stock, or you can fill in yourself with gps readings.
Seems to be working smoothly, and weighs an ounce or two.