Did it just start now?
The speedo could’ve gone faulty. In that case, how is the steering on your car? Does it feel “heavy” at low speed as if it’s not working?
There are a few other possibilities. First is a bad signal from the VSS to the ECU and ultimately, to the Odometer. Check for grounds on the Speedo, near the VSS or by the ECU under the wooden foot plate on the passenger side floor. A badly grounded device can catch noise from surrounding devices, and it would throw false pulses to the Speedo.
The other possibility, if you say the car is completely stock, maybe one of the last owners had a problem with the tach, speedo or power steering before and got a used VSS (Vehicle Speed Sensor), however, the replacement VSS that he got had a gear for the TT and not for the NA. The only way to tell is when you remove the VSS, is the gear Blue or Red?
Blue is for TT
Red is for NA
Those colours only apply to a Manual car BTW.