I agree about GPS. It's easy and quick to swap between bikes, but it's not really accurate or consistent (especially under tree cover). Throw in Strava and their lazy segment start/finish setup, and it's almost useless. I can do the same 10-15 mile ride with somebody going to same speeds, and our GPS can disagree by couple of miles and a few hundred feet of elevation. My old Garmin Edge 705 was more accurate (based on road mile signs etc.) and consistent than the newer GPSs I've owned, but it's a brick.
What sort of speed sensor do you have?