[Roverlike]

I would disagree. Speedo is calibrated to have discrepancy of - 1km/h at low speeds, but as you climb to higher speeds, it would have much greater discrepancy, of even 10% at highway speeds. From my point of view your speedo is reading correct.
In case of factory setup, including tyres, your speedo will always show greater speed then it usualy is.

With change of tyres diameter you might have greater descepancy then before.

GPS reading is also with discrepancies, because public GPS must not have precision like military one. So GPS reading will be more correct than speedo reading, but still it will not be accurate.