Map My Run - HELPPP

only2bfit

Is map my run accurate? While I was almost finished with my run today it told me I was over 2 mi.. then I got home and looked at it and it says 1.6 mi.. It also told me I ran my first two miles of my run yesterday in 8 mins, then my second two in 9 mins.. then I got home and it said I only ran like 3.5 miles instead of 4. Its been doing this every time I use it and I'm not sure if that's just how gps gadgets work or if I should start using something else? I don't wanna think I'm running farther/faster than I really am!