So the digital speedo in my gta is about 5-7 miles an hour off the car has the factory 9 bolt rear in it and the 700r-4 transmission. Was wondering what would cause this and how to go about repairing the only thing I can think of is the tranny was maybe swapped out of a 305 car and the plastic gears that the speed sensor runs off are the wrong gears?

yep, plastic gear
Check out Classic Industries on here, they have em

How will I know which one is right for my car? Also there not that difficult to replace right have to remove tail shaft of tranny to get to if i'm not mistaken?

Correct, the replacements are are color coded, blue is 45 teeth, Green is 34 teeth.
Not sure which one you have/or need.

15-20 Bucks ea.