Be be absolutely fair there will never be a standard amongst the community when held mostly online, it's too easy for any John doe to tell his computer he is using 27.5 inch wheels when he's using 26, a GPS is accurate in the open air enough that I get readings from it that match my speedometer in my truck +/-2mph, but as mentioned going down a hill is probably going to skew a reading a bit.
I would say that both a computer and a GPS app, if used and information presented is done in good faith, is good enough, if you want 100% accurate results go to a racetrack or get a friendly cop/whatever person with a radar on a cold day and get some results.