[Mk2] [Turbo] Rev counter under-reading

[Mike]

Hi folks. My rev-counter has developed an annoying habit of under-reading, probably displaying about 50% of the likely true RPM of the engine. I suppose I have two questions:

Is this common (had my tubby for 15 years and never heard of this personally) and therefore a well know fix? Or, should I just change the instrument binnacle?

If its a case of changing the instrument binnacle, is there a straight-forward way of adjusting the mileage? Now I know that sounds dodgy but my car is knocking on for 150k and I want the mileage on the replacement unit to be true especially if I come to sell the car, I don't want anyone worrying that its been clocked.

Thanks

[jon]

You can just swap out the rev counter for a replacement and not touch the speedometer.

[Mike]

You can just swap out the rev counter for a replacement and not touch the speedometer.

Can you? I did not know that. Could be a solution.

[Mike]

So, I've now fitted a replacement rev counter. However, now when I turn on the ignition the needle immediately jumps up and sits at around 3k rpm before starting the engine. Any ideas why this could be the case?