ipod nano fitness accuracy

Hi,

I have a 7th gen iPod nano and I'm pretty sure that the Nike fitness software overestimates my speed and distance. Furthermore I have noticed that statistics vary a lot depending on where I do hold the iPod. If I keep it inside the pocket of a belt I get a slower speed compared to that of when I put it directly into the pocket of my shorts. Anybody has noticed this ? Where should I keep it while running to improve accuracy ?