Here is another variable. If the gas yourbuying has 10% ethanol it raises the octane.The 87 octanegas with ethanolthat i pick up at the tank farm is at least 90 octane.

I know this because when i need 89 it no longer blends the 93 with the 87.

I work for a large chain convenience store in PA.

 

The 4.7 recommends 87, read the manual. For Hemis, suggests 87-89, any anthing over that is a waste of $ unless you running a tune. Tells you that flat out in the manual also. So running 87 is fine. My mpg dropped roughly .1 mpg switching and the truck ran just fine. Truck wont get upset if you switch, it adjusts to what kind of fuel its receiveing. May seem like the 87 burns faster since you switched when prices went up...thats the ethanol, it burns faster..