Ever since I bought my truck, the fuel gauge has always been screwy. When it is full, sometime it will read full, most of the time it reads 3/4 and occasionally it will drop as low as half. I always fill up when the gauge reads about half, and I have put anywhere from 4 to 15 gallons in.

Any ideas on how to fix this?

 

Test the gauge and wiring at the tank connector with a variable resistor...Test the grounding unit (sometimes misidentified as a "sending" unit) out of the tank with an ohmmeter......Ben