ok its an 84 Z28 with a T5 and 3.73 stock. It has a 140mph speedo, I know the 140 speedo wasn't until 86. so what do I need to make the speedo read correctly. Are the speedo gears/VSS different from the 85mph and the 140?

Plug and play. I'd advise just swapping the speedo head itself. Consider the contents of the following link:

https://www.thirdgen.org/forums/faq-...-fix-pics.html

JamesC

as long as it's mechanical it'll hook right up and you're good to go. here's a link to a company that will reprint and recalibrate guages.

Speedometer Repair Tachometer Clock and Gauge Restoration for Musclecars and Vintage Automobiles

A couple guys on the board have sent in there 85 speedo and had it recalibrated and reprinted to 140 or 160.