450SLC SPEEDO READING SLOW

[johannes450]

I have a euro 450 SLC 1978, the problem is that the speedo reads 20km\h slow right trough the ranges at indicated40km\h i am actually doing 60km\h and at 100km\h i am actuallu doing 120km\h.

What can cuiase this the cable looks lubricated and is not making any noise, the speedo needle is steady and not jumping, it is the correct speedo for the car according to the partnumber. So what else can this be??