Bricks don't get good milage, especially around town. Also, you probably occasionally accellerate hard and use the extra power you have. Most people I've talked to get about 9 MPG with Burbs. I had a 1968 F250 that got that milage in town with a 360 CID engine and the same milage after I installed a 300 horse 500 footpound 460 CID engine. The most I ever got with either engine was 14 MPG. The aerodynamics of your panel should be similar.
