odometer precision?

Hi there, what is your experience with the precision of the odometer functionality? i have tried different GPS (Ublox Neo 6m als well as LC86G) usually with pretty good hdop using the open-rally-computer. but i have an almost 15% lower distance reading compared to the reality (measured from gpx track). is there anything i could check or improve in the code? Any idea would be great! thanks a lot