Speedometer needle is stuck in an upright position

At around the 50 mph mark however sometimes it shows 70 or even 90 mph and this is when the car is at rest, keys out and locked up. When driving it moves between 40/50
up to 80/90 mph. Everything else is a okay and the mileage counter goes up 1 mile for every mile traveled as it should do. It was suggested that the speed sensor was probably at fault but, if that was true it wouldn't explain why the needle is stuck in the range it is especially when the car isn't running. I have scanned it for fault codes and there aren't any, I presume an inoperative speed sensor would generate a fault code due to it being an ECU managed system. A local mechanic has even said that the problem actually originates from the automatic box and the fix involves box removal by a specialist but I think that was rubbish because, yet again it doesn't explain why the needle is stuck where it is. A speed sensor issue would indicate a constantly open circuit when the needle shows 50 mph at rest and apart from draining the battery, the ecu would think that the car is travelling at X mph when it isn't moving and theoretically the mileage would go up at the same rate as well. Is that a correct assumption do you think? Or, is there more likely to be a much simpler explanation like the speedometer stepper motor? I would be really grateful for any assistance. Thanks.