The auto parts store offers two choices. which one do I go with on my 91 z28?

 

What transmission do you have? What parts store and part numbers did they offer?
There is a difference between manual and automatic VSS.

 

I have a 99 4l60e trans in a 91 350 5.7 tpi wiring car. The parts store gave me a duralast su1024 speed sensor which i finally got to picking up this week. hope to fix it this weekend. just not sure if i need something for the factory l98 car that the rest of the drivetrain is or something for the 99 4l60e trans??

 

The SU1024 looks like the default rotary automatic transmission sensor for a 3rdgen vehicle (700R4). Did they look it up from your car year model?
Because your 1999 4L60E uses a different way of generating the speed signal, you will need part # SU1089. This is the passive magnetic sensor for 4thgens.
However, the new sensor in the new transmission generates pulses much faster so your speedometer will peg as soon as you start rolling.

You will need some kind of a frequency divider to adapt the high-frequency output from the 4thgen sensor to the 3rdgen speedometer. Hopefully I can be of help. Let me know if you need one.

Lou

 

Plus on a '730 TPI ECM, the VSS signal goes to the ECM first then to the speedo itself