Excellent post ElanC. While I am not ready to completely discount people's claims that changing tires/rims greatly impacted their FE, I think we can get from this that we must appeal to something other than rotating weight to explain most of it. Also, car tires are not uniform discs, most of the weight is concentrated at the outer areas, still that could not more than double the energy cost over linear mass if all the weight was concentrated at the outer rim.

Things I could think of to better explain the reports that have come in:
o - heavier tires could account for about 1mpg if they were a lot heavier per tire.
o - larger tires confuse the computer, 2" dia more is an 8% loss or 4 mpg out of 50, and this is a virtual loss.
o - Loss of LRR could account for about 3-4 mpg according to reports.
o - increased wind resistance, no clue what that could account for.
o - exasperated people exaggerate problems, could double the size of the problem. :-)

So the hypothetical report of a 2" larger tire, 10-12 lbs more per tire, and loss of LRR, would account for about 8-9 mpg down from 50, and maybe 7-8 down from 45, that is pretty close to some of the bad reports we have seen. :-)

- Kurt