L/U will help about 200 rpms, who drives 55mph? I've done it in the past with 3.91s city and 30–40-mile trips at freeway speeds, if you want to take a long trip (Power Tour) have a 2.76 3rd member ready to swap in, will take 45-60 min. The car I'm working on now will have more noise/heat insulation, so I have an opportunity to hear my wife talking.