the tachometer will always dissapear when you start the car, as its resting position is out of sigh up behind the edge of the lip.
As for the MPH not moving, is it not moving AT ALL? or does it move sometimes and other times it sticks?
If its not moving at ALL, it could be:
busted needle/needle springs
There might be a speedometer sender gauge (i belive there is one in the inline 6's)
Electrical fault somewhere, although from reading my manual the speedo,rev counter, fuel meter and tachometer are on the same circuit so if it was electrical, they would all fail at the same time if there was a dodgey wire/break in the circuit, like on my bloody car!
At my 1st guess on the speedo, id say the springs underneath the needle may have been messed around with.