which is more accurate my fitbit surge or mapmyrun gps???

[AGSM93]

So yesterday I went for a run and wasn't sure how much I ran! Accoring to my mapmyrun app, it said I ran 7.95miles whereas, my Fitbit Surge said I ran 7.50miles. So I don't know which to believe...what to do I go with? 

[GershonSurge]

I've found my Surge is quite accurate. Sometimes, it varies by about 50 feet a mile. 

[USAF-Larry]

There are so many factors affecting the accuracy of any GPS device, that it is unlikely that any two of the will agree completely.  The U.S. Government standards on GPS specify that a GPS receiver be accurate within a 4 meter range (If you want get really bored some time, check out gps.gov and go to the Systems/Accuracy tab). That is a minimum requirement, but if one device is accurate to within 2 feet, and another is accurate within 10 feet, that can become quite a difference over several miles of movement. Then, to complicate things more, obstacles (trees, buildings, bridges, etc.) can interfere with the GPS reception for a brief period of time and the application has to extrapolate the distance between the last time it had a connection to the next time it gets a connection.

And, then, the Fitbit uses steps to calculate distance, which depends on the defined stride length in the setup. The exact stride length, though, varies greatly with speed of the walk/run, the steepness of hills up or down, etc., so it will never be exactly what is defined in the setup. 

The 5% difference you saw between the Mapmyrun and the Surge is probably within the margin of error for all of the sensors.