I am looking at an 80 model that the speedometer is not working. The owner had it checked out and was told it was the cable. But the odometer still works. I did not know that one could work and one not.Do they operate separately on different cables? Thanks

ALSO:
What is acceptable compression range on each cylinder?

 

There is only one speedometer cable, and that spins all the cogs in the gauge. If the tripometer and odometer still increase, than the problem is inside the gauge itself.
As for your compression question, the Factory Service Manual shows a standard of 171 psi, and 128 as minimum. All this information is available for free at: XenonS130 This website has every factory service manual for every year of ZX
I am going off the 1983 FSM, but I would imagine the compression didn't change that much, if at all over three years.

 

I have a cluster if you are looking to replace yours!
I have one in ebay though!!

 

It's likely the gauge itself if the odometer is still working. There is just one cable going to the cluster.