If you buy a Garmin get a Magene S3+ Speed Sensor off Amazon for $20. Garmin has 1 but it's $33 yet does the same thing. The Magene paired with my Edge 500 & Instinct watch (not a very good watch). Both Garmins under calculated my mileage by 13% on the average ride ** even though the Instinct watch tracks 2 GPS systems. Honestly ALL cycling GPSs Garmin sells should just include these because the GPS calculations are so far off.

** DISCLAIMER: these numbers are based on mountain bike rides. Using the Garmin on a flat road ride might be more accurate but I have not had enough rides to be sure, will be testing it in late April at the NC coast. The more hills you have the further off the GPS numbers are gonna be. If set up correctly, the data using a speed sensor is spot on. $20 is cheap to me for more accurate information.