Other than cosmetic changes, the Crown Vic’s/Grand Marquis are relatively unchanged from 1992-2008 (2006 being the last Crown Vic). The motor is exactly the same = 229hp 4.6L modular V8, same trans, same rear gears, etc. The 2003’s have aluminum front crossmember/suspension components. I could see a marginal difference in the '03 & up cars due to the lighter weight, but not 13 mpg difference.