Re: Speedometer error

[Mike Wattam <101714.1343@domain.elided>]

Thanks for your kind remarks.

The mileage recorder is mechanically driven by the speedo cable, so should
be somewhere about right - but is not accurate enough to judge
speed/distance by accurately.

The 'speedo' part of the instrument has a rotating magnet attached to the
cable, and turns an armature attached to the needle.  The faster the magnet
rotates, the further the needle turns, thus giving at least some impression
of speed.

The problem is that the magnet, despite being of the 'permanent' type, does
lose some power over time.  Conversely the armature/needle spindle should
have less inherent friction as time goes on, so one therefore in theory
compensates for the other.  However in practice this does not tend to be
so, the armature spindle in particular can get sticky especially when the
car is used infrequently or if subjected to condensation, hey presto low
reading speedometer!

Please don't ask me why this crazy system was used in the first place!

Mike Wattam
Triumph Stag Register