Did you touch the needles when the cluster was out?
If so, remove the cluster, pull the needle on the speedometer off.
A fork works well. It will take some time to get the needle off. Pull it out gently, until it's taunt, and then yank on the fork that's holding around the metal bar holding the needle on.
It won't move for a long time, and then it will just pop off and flay across the room.
Now go back to the car. Plug the cluster back in.
Turn the car on.
Now, since you're just sitting there we know you are measuring 0mph.
Carefully put the needle back on, BUT keep it just below the catch pin on the cluster (the pin the speedometer needle rests on). Mount he needle on the cluster and then lift it over the pin.
Take a ride and see if it seems right. Following a car that has no modifications and a perfect speedometer might be a good idea. Have them go to a pre-set speed and measure what you're reading. It should be pretty spot on now.
Then put the clear face back on the cluster and re-install the entire cluster into the car.
I had to do this yesterday because my odometer suddenly stopped working. Thank god I had another cluster so I just swapped the guts of the speedometer assembly out.