I should probably know this, but I don't. I'm usually interested in my mileage more than any other metric. Do I need to have a speed sensor enabled to calculate mileage or will the GPS in the 520 do it by itself? I have 2 bikes and I can connect and move the 520 from bike to bike, but only 1 bike has the sensors on it and I don't want to be moving them back and forth and/or possibly have to buy a 2nd sensor.
Thank you. There are a few things I do not understand so possibly you can help again...
Then what exactly is the speed sensor needed for? I went for a ride today with only the head unit and it was calculating speed. I understand that the cadence sensor is required if I want to track and record that metric, but what about the speed sensor?
GPS alone won't give you 'accurate' distance nor speed all the time. GPS struggles with ascent and descent calculations and also if there is overhead cover and switchbacks. Tis why I use a sensor on my mountain bike. If you want accurate distance and speed measurement then use a sensor.
Don't forget to calibrate the sensor - a speed sensor basically counts the rotations of the wheel it's placed on so make sure that your device has a calibration matching the actual circumference of the wheel to get accurate distance and speed readings.
Except for mountain biking where poor reception due to tree cover and terrain combined with switchbacks distorts the GPS track, the overall GPS distance is very accurate. GPS position errors cause short-term distortions but they tend to cancel out over distance. The short-term distortions can cause speed fluctuations, the auto-pause to come on when going slowly and makes the gradient readout less accurate. If you are road cycling and don't care about those things there is no need to have a sensor.
