Recently bought Dart Rally Blacktop, 2.4, Powertech 6. Only have about 350 miles on it. Terrible mileage (about 16-17 in city driving). The wheels are 18-inch x 7.5-inch. Wish I hadn't gotten it, too late now. Can I improve the mileage by changing the wheels? Will that void the warranty? I know the mileage will improve with break-in, but by how much? Would appreciate any helpful comments. Thanks!