The Nike + default calibration is very accurate, but for maximum accuracy you can calibrate Nike + iPod to your natural running or walking style.
Calibrate Nike + iPod for running and walking using these simple steps:
The iPhone can sense whether your workout was a walk or run and correctly uses the new calibration to override existing data. Even after calibrating, the accuracy of the distance measurements may vary depending on gait, running surface, incline, and temperature.
You can reset the Nike+ Sensor calibration by clicking the gear-shaped Settings icon on your iPhone or iPod touch home screen. Scroll down and select Nike + iPod, then click "Reset Calibration" at the bottom of the page.