If you messed with the hanger inside the gas tank recently unscrew the top of it and carefully reposition it .. my car had that happen when I bought it, the idiot thought the sensor was bad .. all I did was open it up, spin it a tad and wha la !! Like new ..
Assuming you didn't screw with the fuel needle or your fuel tank/pump, $10 says when you installed your fancy Tach and Speedo you created a bad solder joint on your cluster, one that is related to the fuel gauge.
Why you ask? Because I have the same problem with my fuel gauge. When it gets hot out (eg 80F and above), my gas gauge will basically stay at the last position it was at before the cluster got too hot. After a while, I can push on the cluster, next to the gauges and the needle will begin to fall slowly to the actual level. Presumably, my force on the cluster is reconnecting the bad solder joint. Once my car is in the garage and cooled down, the gauge wil move to the actual level. My low fuel light works all the same though. When it's not hot out, I don't have this problem. I assume that the thermal expansion of the circuitry has wrecked havoc on the solder welds.
Unfortunately for you, I haven't bothered trying to find the bad connection in the circuit board because it's not that big of a deal to me right now. If your gauge has stopped working completely, "ruh row ragggy". Time to pull it apart and go to town with a soldering iron.