The Chronometric speedo, as the name implies, works in a manner similar to that of a clock. As a result, it is highly accurate, provided of course that the gearing to drive it is correct. On Ariels, it uses a flex speed (speedo cable rotation at 60mph) of 1610.
The PA speedo works on the governor principle but can be surprisingly accurate. It relies upon a small coil spring to determine the movement of the pointer and these springs vary depending whether the speedo was originally 60, 80 or 100mph. Most 60mph PAs went into cars and had a flex speed of 3360. The Ariel ones had a flex speed of 2240. Logical really, given that in the 1930s a car's cruising speed might be 40mph whilst a bike could do 60mph easily enough.
I have a number of PA kits available now and I hope to be able to bring some for sale to the Annual Rally in Pershore, providing that I can get them put together by then.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.