I don't know what your combo is, but I have a 2002 Ram 1500 van 5.9 with 3.55's. 255/70/15 tire. The van gets 11mpg. Mine is a high top conversion so I think the limiting factor is the giant sail effect.
It has the Hughes baby cam and EQ heads, magnaflow exhaust and cat. After the rebuild and add ons it still only gets .5 mile per gallon better than the old factory stock all oem van with 155k.
Even switched to electric fan, still no difference. I am about to rebuild the rear end and thinking of switching to 3.92.
2001 3500 5.9 4.10 gears 245/75R 16 tires, I get around 13-14 mpg now. 186,000 miles, new chain, fixed the gasket on the bottom of intake.
