I am going to swap my 350/TH350 combo out for a diesel and TH400 or TH700R4. The main reason is that gas price is horrible ower here (5.66 USD/GALLON), but also my TH350 passed away the other day (RIP) so now I have to start doing something. Now I have few options. My truck has D44/12 bolt with 4.88 and 38" tires. I did change the truck for 44" but havent bought them yet. I can purchase a 6.2 with TH400 and NP205 or with TH700R4 and NP208 (same as is in my truck right now). I can also get D60 and 14 bolt FF with no spin and 4.56 or 14 bolt SF with Air lockers (ARB?). I know that TH400 is much stronger in OEM form than TH700, but if I chose TH400 and 4.88 then my engine will be at ~2200 rpm at 60 mph. If I have TH700R4 my engine should be turning ~1600 rpm at 60 mph. If I should choose TH400 and 4.56 then my engine should be at 2080 rpm at 60 mph. If I will stick with 4.88 then I will probably use my current D44/12 bolt combo untill I break it but if I will got to 4.56 then I will upgrade to D60/14 bolt. *** So my questions are: How much affects it will have on my mpg if 6.2 is running at 2200 or 1600 rpm. Would the 6.2 with TH400 and 4.56 be horrible to drive? I know that 14 bolt FF is a lot stronger than 14 bolt SF but I like the idea of having a selectably lockers. Would is it that much weaker that I would regret heading the SF way? Thanks, Jon H.