Speedo runs at half road speed.

[Typhoon90]

The speedo on my 90 is EXACTLY out by half the road speed, so if the car is travelling at 80 km/h, the speedo reads 40 km/h. I also suspect the tacho is inaccurate. It seems to me this exactly half thing must be an electronic fault, an amplifier not amplifying, or a speed sensor that's lost a magnet or tooth. The gauges are the late conventional jaeger gauges.
Anyone seen this before and have a fix?

Regards, Andrew.