The previous owner of my Mazda Astina (323F in Britain) fitted smaller profile tyres (195/50 15 verses 195/55 15).
I checked the speedo against my GPS on the motorway and it reads about 4.5% fast, as expected.
I checked the odometer against a measured 5 kilometer "speedometer test" sign posted on the motorway and found it to be accurate.....Huh?
I checked the odometer against the GPS driving to work. The GPS said my commute is 51.9km, my odometer said 52.0km, close enough to perfect. Again I say.......Huh?
My point is, don't assume the speedo accuracy reflects the odometer accuracy, as I always did in the past.
If you really have to adjust your speedo, this might help.
Speedo Corrector MkII Kit "Kit Back Catalogue" - Jaycar Electronics
I haven't tried this kit myself, but it sounds OK.
