Recently, I was having an issue with the fuel gauge On my ?68 XR-7. It wouldn?t read above 3/4 of a tank, no matter how much gas was in it. I figured that the gauge voltage might be a little low, so I checked the voltage, and to my surprise it was set at about 6volts (instead of the normal 5 volts) I reduced the voltage to 5 volts (I have a variable voltage regulator for my gauges) Now the gauge only reads between a half and 3/4 of a tank. I am using an electronic gauge testing device that is designed to give accurate gauge readings ( on vintage Ford vehicles)by bypassing the fuel sender, thereby eliminating it as a source of the problem. Does it sound like I might have a fuel gauge that is not properly calibrated? If so, how do I calibrate it?