I'm a bit confused by gear ratios and such, so just gonna ask my question.

I'm assuming I have stock rear end and tranny. My rear tires are p255/60r15s.

My speedo is off, wife says around 40mph I'm actually doing 50, and around 55 I'm pushing 70. Between 45 and 60 my speedo floats.

First, I'm wondering if I need to change the drive gears(cause of tire size change), I'm not sure if any of the POs did. If I do need to change them, what would I get? The GM chart listed on tgo doesn't show my tire size.

Second, with the floating, I'm guessing either my head is shot or my cable is flapping around, is that right?

 

Can't worry about the error until you fix the floating. Try replacing the head and cable. Then if the calibration is off, we can attack that.