An FWIW comment.
Because the original speedo on my 1979 Guzzi was getting a bit "all over the place", I have been running it with a little GPS speedo. The original was always optimistic though the TRIP METER was always close to spot on.
However the MoT men were less than happy with GPS only.
So fitted a sensor driven speedo - gearbox sensor. Apparently this puts out 1 pulse per metre.
Speedo so configured and it matches GPS exactly on new (barely worn anyway) tyres.
I said FWIW.
