Yes. But probably not by much (outdoors).

I don't use the speed sensor with my Edge 800 or Fenix 3. The two of them are remarkably close at the end of each ride, it's typical for them to differ by about 1/20 mile for every 10 miles traveled.

If you have a speed sensor, your Garmin will use it to determine speed and distance; if not it'll fall back to GPS. You don't have to do anything to make that happen.