Head unit accuracy

[Tenkaykev]

But how did you know it was "twelve miles"? Had you measured it with a steel tape or (less accurate, especially over the Purbecks) a measuring wheel?

My post was meant to be tongue in cheek 😊
We had three or four regular runs over the Purbecks. We called them the 12, 16, eighteen and an occasional 22 milers.
With the advent of gps watches we realised we'd been overestimating the distances, possibly due to the terrain.
The closest to reality was the 16 which was just a tad short.