Anyone able to help with this please ?

Series 1 RS Turbo my mate has had it 8 years.....never missed a beat.....was running fine, sat out in sun today, and thought he would go for a spin.

5 mins into it....the rev counter sticks at 2k RPM....he taps the dash to see if its just sticky....and it frees off.....but now its only idling at 500rpm.....and the revs are only showing HALF what they should be....

i.e. If it should be 3000 rpm its only showing 1500...

Car is running perfect though, idling fine, runs fine, accelarates fine....

Rev counter is just very low on idle and driving though, about half what it should be.....

Speedo is fine too....

Any ideas ?

Michael