how do i figure out what my original gear ratio was and if i figure that out how can i figure out the gear i need to make the spedo work with the 373's

i went from a 2.73 open to a 3.42 4th gen rear in a 92 firebird what drive and driven gear do i need my speedo is off by 12 from the start then goes up the faster i go would like to get that fixed asap

i didnt get anything form thoes other posts im still confused