The difference in measured distance between my cycle computer and Strava is pretty substantial, around 2%. Since I have a wired computer from a brand I have had only good experience with (Sigma), and I measured the tire circumference carefully when setting it up, I'm inclined to trust the computer instead of GPS. I mean, 2% of the measured 2220mm circumference (which also corresponds to the figures cited online for this tire size within a few mm) is 44mm - it's virtually impossible to be off by that much. The accuracy of speed measurements aren't that important to me, but such a difference in distance would translate to almost a whole century over my yearly mileage.