RVC Rev Counter

[alawrence10360]

When bought the car it had a nice installation of rev counter , water temp an oil pressure in the drivers glove box.
The rev counter had never worked apparently as it was an RVI type that is fitted in series with the coil and is unsuitable for the electronic ignition which is fitted to the car ( lumination basic unit )

I have now aquired an RVC type which is fitted in parallel with the coil and is suitable for electronic ignition I understand.
The rev counter works... however at a guess it is reading half revs .
The rev counter is a Smiths RVC 4cyl unit from a 1500 Midget/Spitfire. Any ideas why this may be ?