My Garmin 705 is set to one-per-second recording. Here's a ride that headed north, up the hill, and made a right turn on the return leg. You can see the distance and speed at each one second interval. It's pretty accurate, even showing which side of the road I was on.

If I'm riding downtown or with tall trees near the road, the limited satellite signals affect the position calculations, and the route drifts off the roadway.

15 mph is 22 feet per second, so the recordings would be that far apart.

Mapping from My Tourbook, free software.