I have a Garmin edge 520 on my 29er mtb. Also I have the cadence sensor on the pedal arm and the speed sensor that attaches around the hub of the rear wheel. My issue is in the last 4 endurance rides I have done it has read around 4 to 8kms short(depending on length of race.)last race was 39kms and mine read 35.8. I do swap it between a 27.5 in indoor trainer and my 29in mtb. I have had the speed sensor set to auto calibration.
Any other ideas greatly appreciated.