just bought a 68 cutlass conv. speedo is off about 10 miles faster than the car is going under 60 mph and maybe about 15-20 mph when you get over 70 mph (i'm doing over 90 on the speedo to keep up with fast interstate traffic). it also starts boucing a little bit when you run it over 90 mph. it has a th400 with i think 3:08 gears in a limited slip 12 bolt. tires have been replace with 15" and the size is 245/15/60. guess the gear needs to be replaced. i have never done this. how hard is it to do and is there a way to tell which gear i need without taking out the old gear first?