New Tires Mystery

After reading so many threads on tires and fuel economy and nothing really seeming scientific or conclusive, i decided against LRR and instead to get the tires in my budget that had the performance I was looking for. Since inflation, tires have seemed to have gone up 50% where I am in California, so even a set of budget tires cost me $560 total. Because I could not find adequate information and most tire threads are full of anecdotes, I just decided to roll the dice and go with Kinergy ST H735, tires that no one really recommended for the Prius.

Before I was getting about 40.5 mpg at 40psi. 42mpg if I drove conservatively, but the way I normally drive it averaged 40.5. I got the new tires and waited about 1500 miles of break in to start clocking the mpg and tested the psi for a few hundred miles, all driven the same.

At the standard 36 psi I was getting a slightly disappointing, but expected 38 mpg. Lots of people said to expect a drop off if you didn't get LRR's so I was actually ok with this drop.

Here's the weird part: I decided to bump the psi to 40 and the mpg went DOWN to 36mpg. I have no idea why this happened. The ride comfort was terrible too.

Then I tried 38psi and it went up back to 40.5mpg.

Anyways this is just another anecdote, but I figured I'd give my experience with the Kinergy ST H735, which I'm now very happy that I got no drop-off in mpg.

36psi = 38mpg
38psi = 40.5 mpg
40psi = 36 mpg ????

Thoughts? I can't explain the significant drop with higher tire pressure.