I had 3.42, but just had them switched to 3.73. The 3.42 got about 1 mpg better in steady highway driving at 75 mph. The 3.73 helped accelleration and pulling power (I pull a 2200 lb. boat and a 2000 lb. enclosed trailer). Around town mileage is about the same. I average about 16 mpg with the I-6 engine in my 2006 model.