This morning I tried Speedmeter while cycling to work. The current speed seemed to be accurate in the most cases (difference to actual cycling computer usually 1km/h).
I travelled my usual 8 km to work, but the distance it said was 12.363 km. The average speed was 28 km/h because of this, the actual average was 19.55 km/h.
It stated the Accuracy would be 2-5m most of the time.