1994 Pickup DLX 3.0 Auto Trans. 4WD.
New speed sensor installed first. Odometer works fine. Speedometer doesn't register, but after driving a short distance it will start working.
Weird part is when I use a turn signal, it will jump to zero on the first blink, then return to normal speed. It's fine until I use the other signal,
and it does the same thing. Bad ground? Help, I'm confused.
Only other issue is that I just filled it up but the fuel gauge won't go past 1/4 tank.

 

The turn-signal-jump thing happened to me. It turned out to be a bad flasher relay. Get an electronic flasher relay, should go away.

As for the fuel gauge, probably a bad sender unit. There's an article I saw (maybe on here?) that showed how to fix it.