Thanks!
I did not realize that my Garmin 820+ connected to the Wahoo speed sensor ONLY by ANT+ and Cyclemeter can pair to the same Wahoo speed sensor (WSS) by Bluetooth concurrently. So I have paired the WSS to both the Garmin and to Cyclemeter. But on Cyclemeter I have turned off "Speed Sensor Failover to GPS" (under Settings/Bike) because I would rather have the WSS calculate the speed. The wheel circumference on both the Garmin and Cyclemeter is set to 2103mm, which usually works great on the Garmin - we'll see if that improves Cyclemeter's accuracy. The Garmin usually calculates overall distance to within 0.1% accuracy if the wheel circumference is set correctly - I set the wheel circumference manually, based on experimentation.
So we'll just have to see if this revised setup gives Cyclemeter more accurate speed and distance readings, that agree more closely with Garmin.
I live in Palm Harbor and there are mile markers on the Pinellas Trail. My usual trip is from mile marker 30 to 44 and back. The Garmin has been dead accurate in calculating distance between the mile markers, but Cyclemeter has often been off by anywhere from a hundred yards to a quarter mile.
I have a RFLKT and it worked great, but would often crash and complain that it needed a new battery, and the only way to reset it was to open it up (four tiny screws), flip the battery over for ten seconds, then reassemble. So I had to replace it with a more reliable device.