Im going to lay some chemistry on you here, Most of what gasoline is Hydrocarbons, which breaks down into propane, methyl-butane, benzene, etc and then there is our good friend Octane C8H18! you need 1 o2 molecule for every carbon and 1 for every 4 hydrogens. the more abundant Carbon and Hydrogen are the more oxygen is burned, the more oxygen thats burned the less fuel is needed to burn that Oxygen.So 93-104 octane fuels (though alchohol enhanced) use the same amount of oxygen with less fuel, thusly increasing your gas mileage. At $2.40 a gallon vs $2.60 a gallon you'll need to see about 1-2 mpg increase to really save more than a few dollars a year potentially even lose a few in the long run if you average about 10000-12000 miles a year, but If prices were back in the $3 and $4 range half a mile per gallon could save you 75-500 bucks a year with those same gains in the 10-13 mpg range. I averaged between .8 and 4 mpg better when I had a premium tune this summer compared to "mileage XS" for winter so those sorts of gains are possible. But understand more fuel is needed the colder it gets because of the dencer air and awd comes into the factor sometimes so 4 isn't a completly realistic #.
