You have to calibrate the foot pod for it to work properly, and this is best on a track where you have a precisely measured distance. Once it's been calibrated it is fairly accurate, +/- .5%.
The other thing of course is that treadmills are notoriously inaccurate, the only way to be sure that the one you are using is accurate is to measure the length of the belt and count the revolutions up to 800m or so.
That should give you the data you need to produce fairly accurate measurements.