Just bumping this with some info.
I'm fairly certain that even if it isn't exact, it'll be fairly close.
I was very, very wrong!
In my original post, I supplied the Phase 1 speedo information from its label. The relevant information is the 16000. That's 16,000 pulses per kilometre ( the IMP after it I think means it's got an MPH dial ). I've confirmed this by attaching a pulse counter to the car and going for a drive. When the trip mileage recorded 1 mile, I noted the pulses that had been counted, converted it to KM and it worked out to be near enough 16,000 per KM. ( Repeated a few times to be certain ).
I've also recently acquired a P2 instrument cluster and this is where it shows I was quite wrong in assuming the signals would be similar.
I attached +12v ( to the permanent +12v & ignition controlled pins ) and an earth to the Phase 2 instrument cluster and a function generator outputting a 50% duty cycle square wave ( 5vpp, 0v base ) to the ABS ( vss - vehicle speed sensor ) input of the instrument cluster. This showed that the P2 cluster expects about 3,500 pulses per KM. That's quite a difference from the 16,000 P1 value!
My retro fitted climate control seems to work fine although it must see the car moving about 4.5 times faster than it actually is. That however is not the reason I looked into this again. I have another project in the planning stage that needs the vehicle speed.
Hope that helps someone in the future.