Big difference is rear gearing. Friend got towing package so it has 3.73 gears. Not fuel freindly even with OD. I could see the 3.55 gear helping out a bit. From what he's telling me(now truck has couple thou. miles on it) that mileage is getting a bit better. It's alot of in town driving so 15 would be pushing it. Overall I think that is really sad, but what are you going to do?