I believe the manual of the 310 states that it can be off by 7m every km. Personal experince has it off about 10m every 5km about 25 percent of the time. More often than not, it is dead on for run courses or local trails. And it can always be the reverse as well, last week it my watch said 6.2 miles when the markers at Shelly Lake said 6.5 miles. At the end of the day, what else do you expect, nothing I use with technology is perfect.