ridewithgps.com
Data from the GPS I'm using (a car GPS) is unreliable and always overestimates. I've tried mapmyride as well but found ridewithgps the more reliable. Strava might work too (haven't used it much).
Don't have a computer on my bike. Chart all of them on mapmyride.com after the fact. I save the ones I do regularly so I can do a quick check and enter my mileage. No idea how accurate it is.