2006 Chrysler 300 Bad Gas Mileage

I recently bought a 06 300C with 19k miles on it. On the highway I get about 20 MPG and in town about 13 MPG. On average for a typical week it gets about 14 MPG driving on highway and in town combined. I use to have an 07 SRT Design and it got combined over 17 MPG for the same commute. It doesn't get near the gas mileage the 07 got. The person I bought it from said the battery died a few times and I read online somewhere that it could take some time for the Multi Displacement System to reset if the battery dies. I'm wondering if there is a way to check if the MDS is working? Factory wheels and tires were P225/60R18 (704 Revolutions per mile) and the guy I bought it from put P265/35R22 (688 Revolutions per mile) which is about - 2.4%. I've also wondered if the change in wheels and rims would "throw off" the MDS and shift points. Any help is appreciated!