3 tanks of regular / 3 tanks of premium

Butt dyno for the most part. These are the same people that say their truck has more power with a new exhaust tip.

 

Octane is in gas to resist pre-ignition. Sometimes the gas will ignite in the cylinder before the engine wants it to. When it does this the engine injects more gas into the cylinder. (the higher the gas in the gas to air ration makes it less explosive, the optimum is 14.7 parts air to 1 part gas which is more explosive than dynomite, and very volatile and easy to ignite) The extra power people claim comes from this as well. The lower octane ignites before tdc and cusions the piston as it goes up, where as the premium ignites at or right after tdc with less energy lost in the cycle. Since the tuning on cars can be so exact with ecu's now there will be little power gain noticed unless you do not properly maintain your vehicle.
As for your experiment unless you were running your truck out of gas between each tank your numbers can be off by as much as 3-4 mpg. The best way for measuring mpg is to fill up each time and measure the gas displaced. While your gallons will be different each time, your gallons used to miles traveled will be exact and leave no wiggle room.

 

i run a tank down to half with 87.....then fill with 93.....then run it half down then fill with 87......just something i do

 

Same