How can you tell what kind of gears is in your car? I have an 87 gt also and i can tell that the gears are different. The speedo gauge is off a little but when i run with traffic on the interstate it tachs to like 3100.
jack up the rear of your car and support it on jackstands. mark a rear tire with chalk,marker, ect and also make a mark on your driveline. count how many turns (by hand) of your driveline it takes to do one full rotation of your rear tire. this will tell you your ratio. 3.27:1, 3.55:1,3.73:1 ect.