DML FAQ : Transmission & Gears : What is the difference between 3.55s and 3.92s?
3.55's would be a better highway cruising gear. They keep the engine RPMs lower for a given speed as compared to the 3.92's. The lower gearing is for grunt. Basically getting the engine into its power band at a lower speed. Overdrive helps mask some of this, but there still is a MPG penalty.