Garmin 55 mileage is not accurate

I just got a Forerunner 55 and the mileage is not recording correctly. The first day I had it I did a run that I knew the milage of - 5 miles. Right away I could tell the mileage was over - the first .5 miles was recorded within the first two minutes of my run, blocks before the actual .5 mark actually was. By the time I was done with the 5 mile loop, my watch said that I had done around 11 miles. Also, the pace is always accurate, but the mileage "laps" are not. For example, my first mile would be maked as a five minute pace, (cause it was five minutes into the run), but then my pace would say 10 min / miles, which I knew was correct. I have tried factory resetting, changing the recording settings, the gps, and more. I even got another replacement from amazon but the same issue is still happening. Can anyone help?