Luc
Congratulations on passing the Contrôle Technique
We had use a GPS sensor for our tripmeter on a rally earlier in the year, rather than the one fitted to the front wheels. We were surprised how inaccurate it became when it didn't have full access to the GPS signals, for us it was when we were travelling in woods/forestry. It 'stretched' distances, and the longer we travelled in the woods the farther out the distance became, which is not helpful when you are looking for a turn at a set distance! It also made the speed read lower than it should, towards the end of the rally we realised that the speedometer and milometer on the car were more accurate than the tripmeter in woods, a great surprise, as we were using a 1970 2.5pi!
Anyway I suggest you fit a wheel sensor, or two one for each wheel, if you can and use the GPS sensor as a backup.
Like you I fitted the trip meter and clock so as to leave no damage to the car, photo attached.
Dave