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The article below may be able to help you with this.
How do I calibrate or recalibrate the Nike + iPod system?
To learn how to calibrate Nike + iPod system, please read "Nike + iPod: How to calibrate the Nike + iPod Sport Kit" or the Nike + iPod User's Guide.
Ordinarily, the sensor does not require recalibration. You may want to consider recalibrating if your fitness level changes or if you would like to try to improve the accuracy of your results.