What all could cause my digital speedo to read 0 most of the time?. Every once in a while it will kick on, and read for a couple miles then go back to 0.

 

Bent / kinked / dry speedo cable... that's the first thing to check.

 

Your cable could be broken, it will occasionally catch, the 2 broken ends making contact and getting some rotations in. If it's broken, you can replace the cable inside your housing, as long as the housing is kink free and not binding. Give Steve a call, he has a cable on his site (page 77), $30 to your door. 300zx Parts For You webpage

You can download a factory service manual, it will walk you through the replacement, I'm sure there's probably a few write ups online.

XenonZ31 Reference

Does your odometer work correctly ? If so your speedo cable o-ring could be leaking trans fluid into the sensor, giving you some pretty erratic readings. Found this post on Nico, check it out Speedometer issues 86 300zx gauge dash : Classic Z Technical

 

Sorry ive been busy and forgot to reply.
No my odometer doesn't work at all, and the car is an 87 turbo did they use a speedo cable
that year or does it have some kinda sensor?

 

Steve has a cable and housings, if your housing is good you can order one from him, he salvages mostly Z31's, there's a link in my post above for the part and that it takes a cable