My 2000 dak ex cab 4x4 3.9 with a topper is getting 3 mpg more than my 2000 durango with a 5.9

I had always heard the 3.9 was just as bad as the V8 but in this case it is not even close, I am happy with my truck, but like my title implys I am in the search for more. Today I put a cold air intake on it, I don't think it will help because the O2 sensor is going to make it dump more fuel in to make up for the extra air. One of thease days I will swap the O2 sensors so the primary one is reading the mix after the cat, that should fool it into thinking it is rich and lean it out a tad, I did this on a 5.2 ram truck and it worked great, the fuel trim went from adding 3% more fuel to takeing away 3% from the fuel map and gave me a consistant 1mpg. If your truck has the 518 trans the A-500 can help because it soaks up less of you already low HP and it can give you some more gear multiplication in 1st and 2nd gear. Another small gain can come from the 8.25 rear, it soaks up a little less power than the 9.25 that a lot of them came with. I pulled the belt driven fans off both rigs and that helps, both of my rigs have electric fans but I have pulled them from my 91 dak and it was fine with no fan unless I sat a long time in traffic, same with my 68 cuda but I have an electric fan on it to turn on in traffic.

I am trying to find info about a computer also so if anyone knows of one I want to know also.