Bonjour Jaques
If the speedometer system is similar to the 4 wheeled morgans, and you have a trip meter on your speedometer to measure the distance on a specific journey, the trip reset button has three functions. One function is to switch the mileometer between total mileage and trip mileage. Secondly, when showing trip mileage, a long press will reset the counter to zero. The third function (this is where your problem may be...) if the button is held in whilst starting the engine, the speedo goes into calibration mode where information can be entered to alter the measurement accuracy .It appears to me that it could be stuck in that mode? Try operating the button a few times or it could be a loose connection.
If none of this applies to you, I am sorry to have wasted your time.
