Fuel quality plays a huge part in obtainable fuel mileage. No other way to explain a 3 to at times 5 mpg difference between tankfulls, when I drive pretty much the same route consistently, and use the same station to fill up, with the cruise control set. But then that's what you get, living in a state that has no real fuel quality standards.
Aside from that, the best mileage I get is when I occasionally run a tank of 89, then when it's down to about a quarter tank, I fill with 87. That tank of mostly 87 does very well with that tiny bit of leftover octane boost.
Pretty much what the title says, all off-topic related posts can be posted here to share with everyone.