the only thing that would help you is going to a lighter tire, which means lower profile with the catch being that the extra metal on the rim has to be lighter than the rubber that it is replacing. also going to a narrower tire or a tire with harder rubber, which will have less wet/poor weather traction but less rolling resistance thus improving fuel economy. Going to a larger O/D wont really help since you will be travelling a greater distance with every revolution but it is harder for the vehicle to turn the extra rotational mass.
essentially i dont believe the gain would be much, though every bit counts with fuel costing what it currently does.
Ya! What I was thinking was a narrower tire, for less rolling resistance and a slighlyly thinner tire to run to lower rpm's, ie. from 265*65R16 to 235*85R16. Just a thought as I have to get a new set of tires, so why not think of improving gas milage.