I used the commonly-recommended SpeedoDRD U1. I'm not aware of any other way to do it. There was a tutorial here that showed a heck of a lot of work, and I didn't understand why, so I started by popping off the windscreen to see how the wires run. Easy. I took off the headlight for easier access, peeled back the wire protector, and cut the wires 2" from the plug behind the speedo. Used the provided connectors to hook it all up, bolted it all back together, and started with a setting of 11.1%. After a I couple test rides I ended up at 9.2% comparing the speed to a GPS app on my phone. I've only been to about 25 in my neighborhood though, and it might need more tuning when I get to higher speeds which would show an error more clearly.
I tied the device to the subframe behind the headlight so I can reach in and program it without removing anything at all.
Maybe the 11% I've seen others mention is with the 18" wheel?
