According to my ScangaugeII, cruising at 30-35mph in 3rd gear (5 speed, 3.55's) yields better mpg than in 4th gear. Same with 55-60 mph in 4th gear as opposed to 5th. Essentially, the higher rpms for cruising are better with low throttle input as the engine has zero "lugging" to it. I get my best mileage at 58-62 mph in 4th gear (still talking about the 2000 2500) and that's at 2200 rpm or so. When I changed my shifting habits to wait until 2400-2500 rpms, I noticed my avg mpg for the tank went up as well. I'm not talking about hammering the skinny either, I'm talking very little throttle input all the way through the gear. Again, I only say this because I have paid close attention to it thanks to the scangaugeii. If your results aren't the same, Im not responsible. In fact, I would advise you to buy a scangaugeii before even attempting it, but I am confident you will see what I am seeing. I never use 5th gear unless planning to go 70mph or more (quite rare).