Robert, do you have a digital multimeter? If so, go to Radioshack and get a 1K potentiometer(thats 1000 ohms). If not, buy a meter while you're there :) Hook 12v and ground to the gauge, then ground one lead of the potentiometer, either end lead of the three. Connect the center one to the sender lead on the gauge. Make the gauge read empty and full by twisting the knob on the potentiometer, and record the resistance readings with your DMM(digital multi-meter). Should cost less than 2-3 bucks and then you KNOW which sender to buy.