I have a '56 f100 and recently installed a new autometer fuel guage. I realize now the guage and sending unit do not match in relation to their ohms operating range. My tank is only 1/3 full but the guage is reading 2/3 full. The guage operates 33 Full/ 240 empty. I know I can buy a new sending unit for $30 bucks to match my guage but I wanted to know if there was another route. Electical is not my strong point but could I use a resister in-line from my sending unit to my guage? Does that make sense? Any assistance or troubleshoot thoughts would be appreciated.
There is no simple resistor that will solve your problem. Not only is the actual resistance range of the stock sender different from the aftermarket units, the size of the range is different as well. If you added a resistor, it would only calibrate correctly to one point on the guage. The error would increase the farther away from that one point that you got. Electronics are available that will scale the signal from the sender to match your guage, but the price is prohibitive. Just go buy the correct sender. There are nickels to be saved elsewhere.
If the tank is in the cab then the sending unit is easily reached and a new sending unit would be the best solution. However, if you really want to pursue the resistor route then you will need to know the resistance of the sending unit currently in the tank. Resistors in series always ADD, resistance of resistors in parallel is always LESS than the smallest resistor. From what you have stated it sounds as if you have too little resistance in your sending unit...just as BOR stated, knowing how much to add is tricky. You could add 33 ohms and make the guage accurate at the "empty" side but you may find that the "full" side never makes the guage read more than "3/4". A combination of resistors in series and parallel may solve your problem but I can't tell without the sending unit resistances.
I know that if I continue my "Evil Life Style," some day when I die, I'll go to hell and this will be my punishment: Satan will sit me at a computer and every week someone will write in about mixing gas gauges with the tank sending unit that doesn't match; or people will want to know why they can't get there signal lights to work properly after installing a used 1985 Porsche wiring harness in their Bonus Built. And it will happen over and over and over.
Question? How much money are you spending, or going to spend, on your truck to make it run or to restore it?