I had to change my motorbike’s speedo

If a vehicle is recent and made in the EU and thus has an EU certificate of conformity from the manufacturer, then it should be accepted and does not have to undergo inspection for homologation in order to carry French number plates.

You can apply directly online for a French registration document in this case.

A car would also need to pass a French MOT test (contrôle technique), including a check on the speedometer, but this should be limited to checking it works.

In other cases you need to apply for the vehicle to be inspected by the Direction Régionale de l’Environnement, de l’Aménagement et du Logement (Dreal).

Each region has its own. Created in 2013, these bodies absorbed the functions of the old inspectors of mines. The link is that the first regulated vehicles were mine trains.

The law is officially the same for all but in practice interpretations can vary from inspector to inspector and from Dreal to Dreal across the country, so the interpretation you experienced might not be the same as that experienced by someone else. Triumph France said it is not usual to have to change the speedometer. The legal requirement is that it be clearly marked in kph and that the kph divisions are numbered in divisions of 10. Therefore a speedometer numbered 10kph, 20kph, 30kph, or one numbered 20kph, 40kph, 60kph, should be OK. If it was numbered in 5kph sections, then it would have to be changed.

There could have been a problem with the legibility of the kph scale in the eye of the inspector, who might have thought the mix of numbers was too confusing, or there might have been another aspect of the original which they did not like.