The “Economy” meter is actually a vacuum gauge plumbed into the intake manifold. Inspect vacuum lines under hood to verify all are connected. Next would be the line at rear of instrument cluster, it has rubber tubing connected to hard plastic line. Rubber fittings deteriorate, thus creating leaks. For diagnosis, it would be worthwhile to test automatic door looks (if vehicle is equipped with this feature). Failure of locking system indicates vacuum leak, which may be related to gauge not reading. Normal operation is: idling provides sufficient suction, causing significant needle deflection. As throttle is opened farther, vacuum drops off reducing reading.