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Mully,
There are primarily three physical properties that affect RR - deflection, volume of the material being deflected, and the hysteretic properties of the material being deflected.
Hysteresis is just a fancy word for the internal friction that generates heat. Anytime something is stretched, bent, compressed, etc., the internal friction of that material generates heat.
In the case of tires, it's the amount of deflection - the difference between the tire being a circular shape, and the shape it takes when in contact with the road surface (basically flat!) Most of this deflection is in the tread area, so the tread rubber contributes the most to RR.
In your example: While the volume of tread rubber would be smaller (narrower tire), and the amount of deflection would be less (due to the higher pressure being used), the tread rubber itself more than offsets any apparant gain.
I just came back from a week long seminar on tires, and one of the papers detailed a coorelation between the various RR tests in use (and under development). Their conclusion was that it didn't matter which test you ran, you could you grade tires by a single point test. What they didn't address was the apparent differences within a tire line - some of which seem to be size related. This means that the goal of having a simple, easy to follow guide for selection of tires by RR is still eluding everyone.
But buried deep in this paper was some RR data of both P metric tires and LT metric tires. I was struck by the fact that the LT metric tires, while having lower RRC's, when tested at the same load as a P metric, had much larger RR values.
I found a formula used to calculate RR in a P metric tire once a test has been performed. However, accompaning the verbage on this formula was a comment that LT metric tires behaved differently and the formula would be different and more complex. This is obviously the difference between what you might think would happen and what actually happens.
Also, I think there is a clue in the data between the 2 tires: In spite of the LT metric tire being physically smaller, it weighs more.
Bottomline: If you are interested in more mpg's, then going from a P metric to an LT metric doesn't appear to be the way to go. (which is one of the reasons I always identify tires with the letters that come as part of the tire size - in this case P245/75R16 and LT215/85R16.)
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