Thanks to all for responding,
I got a lot of useful information from
www.rjes.com Subaru Powered VW Conversions.
It was Richard that suggested 4 pulses per drive shaft rotation.
He also said that the engine management system would accept this and operate happily but the the speedometer would not show true speed.
Currently the vehicle runs fine, just the Check Engine Light continuously stares me down.
The old girl has had a hard week. 4400 km in 4 days with several hours sitting on 150 kph (legally).
As I stated I need to actually DO IT .... setup the hall effect sensor then measure the speedometer error and decide from there.
Crutschow's magic box maybe the solution if I take the lazy cowards path..... which I may....... and there is always Arduino or Picaxe.
In the end it may be a combination altering the number of magnets to match the simplest division/multiplication method.
So I pick your brains while I wait for the bits to arrive.