Thanks for trying again, but I'm still not fully convinced.
What concerns me is that the conclusions run contrary to several world class hypermiles practices. 70-80psi is standard pressures for those trying to wring out the very best fuel economy. I don't think these folks are using those pressures because they like the ride
Instead they have found the high pressures to be effective in delivering on another parameter - fuel economy.
I studied your chart and I see a couple things which don't look quite right. First, there is some inconsistency at the 60&70 psi level. In both cases, there were two significient outliers.
Second, not mentioned in the methology was warming up the transmission(I guess it has one) and tires. By starting cold and doing the tests in a reverse order of pressure, there is a bias injected in favor of the low pressures.
A question that remains unanswered it "Does roll down distance equate directly to fuel economy?" I'm not at all sure we know the answer to that, but hypermiles, by their actions, vote "no."
It is undeniable that tire pressures is a game of deminishing returns, but the real question is where the returns plateau. JMHO, but doesn't seem correct. All kinds of tests can be designed, some of which deliver misleading results. On the other hand of course, is 80 psi over 60 psi better to any significant degree for most folk. The answer is probably "no."