[northwind]

Most bike speedos are inaccurate... I believe it's partly a homologation and legal issue- if you had an eactly accurate speedo then fitted a tyre with a different radius, the end result could be that you unwittingly speed, for instance. Internet rumour says that it's illegal to make an underreading speedo but not an overreading speedo (naturally, internet rumour varies this for different countries of origin, it's always either America, Germany or Japan).

Speedo healer would sort it. Or, run the speedo off the rear wheel, and that will be close to accurate (that's how mine runs, but not for this reason.) Fitting a 120/70 tyre will approximately half the error too.