Just got an 03 Excursion with the 7.3. Right now it has a lift and 35" tires. I'm thinking of dropping it back down so my wife is a little happier and get some regular size tires to help with fuel economy. First question, how do I tell what kind of rear end I have? Is there a code some where on vehicle. Also how far off do 315/75/16 tires throw my speedometer off?