Speedometer problem

[salsa]

New to lat-g today. I have a 1973 Firebird with an anoying problem. The problem is the speedometer is not working right. The problem is, at rest or stopped the speedometer shows 40 mph and when accelerating it does not track right or should I say show the correct speed, it sticks and is not moving how the speedometer is suppose to. The odometer does work properly all the time and at all speeds. I replaced the speedometer cable and no change. So I ordered a new speedometer from classic industries and installed it and it does not work correctly either, it takes quite a bit of speed for the needle to move and does not appear to be showing the correct speed. Is my problem with the speedometer gear in the transmission? Is it difficult to change the gear at home myself? Any transmission experts out there that can steer me in the right direction? Looking forward to your help.