You guys and your switching from 6V to 12V!!!! I don't know if you have a 6 or 12V gauge...a 6 or 12V sender or if it matters or if they can be mixed and matched and you got resistors and amp gauge trouble and you are reversing wires, WOW!. Listen to Tiny, it sounds like he just left the gauge and the sender as 6V. You say the sender is 0-30 Ohm and you tested it. Without getting mixed up with the other posters problems at this time, did you test the sender by itself to see if it reads 0-30 from stop to stop? Do that. Check the "search" for others who have switched to 12V and the various things they did electrically. If the gauge can take 12V and the sender can take 12V (supposedly they can according to Tiny), hook them up outside the truck on a bench with a battery charger or a battery and see if they work. If they don't work and the sender showed 0-30 ohms, when tested separately on a ohm meter, then it's a bad gauge. If they work OK, it's bad wiring hookup/reversed wires/bad ground/whatever!
To bench test the sender and gauge: Back of gauge, the terminals say the "IGN" and "SENDER", hook your power supply pos (12V) to "IGN" terminal on gauge, wire the center tap on sender to "SENDER" terminal on gauge, hook up the case of the gauge to the flange of the sender to neg of power supply. Operate float and observe gauge. Get rid of the ammeter. Put in a voltmeter. BTW: most of the time it's the grounding. You MUST have a dedicated ground wire from a sender mounting screw to nearest frame bare spot...and a good ground from gauge cluster housing to body/dash bare spot and all the other grounds necessary: batt to engine, eng to body. body to frame. (Braided straps)
You could really have a 0-90 OHM sender, your resistor could be changing the ohms the wrong way, as mentioned by Tiny or your float could be leaking
