There is an option to calibrate the Nike+ adapter to my walking and running speed. You select a distance, run that distance while it gathers data, tell it to stop once you know you hit that distance, and it uses the information to better estimate distance ran during regular work outs. Today I spent the first mile of my workout calibrating it to my walk and run speeds. Once I finished the first mile I started a workout to see how well it worked out. Once I hit 3.2 miles I was done, so I stopped the workout on my iPod. Lo and behold it read 2.18 miles! Much much more accurate than before. I can't wait to see how the calibration holds up on my future runs.