So my Speedo on my car is always at zero and it's always been like that but today I cut ground to main relay and it jumps around, but when ground is connected car starts and Speedo don't budge, can someone tell me what's going on with this thanks

 

Probably a corroded speed sensor contact. Pull it off the trans and take a look.

If it can be cleaned, use some die electric grease when you put it back together