At that size, you might get some mileage savings buy going to aluminum wheels, if not already equipped, to lessen the unsprung weight & rotating mass.
For mileage, look for a tire with as little horizontal voids as possible, and 'closed shoulders'...like a 'rib' tire seen on trucks. Traction will suck, but rolling resistance is minimized. You see this feature in many high tread-wear tires and premium touring tires, Like BS's Turanzas.
Not recommended, but if you can find a 155/80/13 tire that meets the load requirement for your car, this is another option to reduce rotating mass. On the down side, smaller tires will brake poorly, you will loose handeling, and might not fit the rim.
Honestly, you may only gain 1/2 MPG it all your effort on tires/wheels. Just git a good quality tire that meets your needs.