Rob - If your Imperial has the EFI, and your mpg readout is 99.9, look for the little resistor, traveling from front to rear, on the left side of the engine, inside the wire harness, it'll be shrouded in a long, rectangular, rubber, insulator and there'll be a kind of lightning bolt embossed on the case. Inside of this is a resistor; it's value is: 10k ohms, there is no polarity end so don't worry about which end goes where. When this resistor fails, it'll read 99.9 on the dash mileage read-out. Hope this helps....Bob Harris