My speedodemeter is off by 8 miles an hour could that be fix just by changeing out my transmission because the transmission doesn't shift right it takes a min.

Assuming you replace it with the same transmission it should be fine. If you've changed your tires to a larger or smaller size it can also effect your speedometer. If this is the case it can usually be corrected by replacing the speedometer cable with one for that particular combination. If your transmission isn't shifting right you should check the fluid level. Also check for any transmission leaks because those can effect the shifting as well.

That want fix your speedometer you said your transmission was bad, replace that also replace the speedometer cable it is located on the transmission and the dashboard behind the speedometer also does it have bigger tire if it does that could make the speedometer show it being off Hope that helps you out .

Tire size usually is the cause. go back to recommended tire size, change the speedometer drive gear in the transmission, or it may be new enough for a power programmer to adjust the engine computer for the bigger tires and get that speedometer back reading right.