well just from my experiences,my old 84 c10 with 6.2 and 700r4would run about 25-26mpg,and my 92 c1500 with n/a 6.5 with nv4500 ran a high of 27mpg on a half dozen occasions.i have since put my banks turbo on it and turned the pump up and man lots more power,yet i have still pulled 27 on occasion.they both use 3:08 gears.so if i had to chose i would probably go 6.2 with the 4500.but i really don't think it will be much more,i've been thinking of a way to hit 30mpg.but just haven't got there yet.