agree with oldog...
a lot of fords in the 60's-70's had a unit under the dash that did this for gauges. was low tech bi-metal resistor. i know my 66 bird has one and they are simple to wire in. the "ohms out" is what i would be leery of as it determines your full and empty readings. btw my car does the same thing with a unbaffled tank and a SW gauge. after a few years i have learned to read between the lines.......
