i think its closer to 20 gallons. My gas light does the same thing. It only will read 3/4 a tank when completely full and the light comes on with about 7 gallons left. I use the trip odometer for a gas gauge. Just go 200-250 miles and fill it up again. Usually takes about 13-15 gallons to fill it back up. Best thing to do is get a small gas can, fill it and the truck up and drive til you run it out of gas. Compare that to where the guage sits thru the drive. That'll tell you how many gallons it is and if the guage is off.