Fitbit-vs-Map My Walk

rondabug

Since I got my Fitbit (Flex), I've been relying on that to chart my distance and calories burned. Today, just for the fun of it, I fired up Map My Walk and used it along with Fitbit. MMW clocked me in with 3.4 miles and Fitbit clocked my in at 2.5 miles. THAT is a significant difference! Which is more accurate?! Which one should I rely on for the most accurate tracking? I have NOT measured the length of my stride for Fitbit. Would that help to bring the two closer to the same?