According to the experts (whomever they may be), increasing octane will not increase mileage and in fact my hamper it unless the engine has been tuned for the increased octane. I am from the era when muscle car engines needed all the octane they could get and then some and thus this line of thought is difficult for me. In modern vehicles calling for 87 octane I have always alternated between the middle grade and premium grade instead of using the 87. This was the case from day one with my 04, and was the case from day one with my 00 as well. After a long talk with Mike Troyer about octane and tuning I filled my “stock” (no fancy air filter, no performance chip, & no after market exhaust at this time) 04 up with 87 (just about killed me since again, my mental model dictated that 87 = bad, 89 = better, & 93 = good). Well low and behold, within 50 miles I noticed my mileage increasing according to the Info Center and the truck actually started running better. In addition, the truck was easier to start. This was my experience and may not be what is experienced by others.
As far as your mileage goes – I have no idea how you drive, IE, whether you peddle the gas, ride the brakes, punch it every time you leave from a dead stop, cruise at 80, etc. I do non of these things, well I do punch it now an then, and get over 4 mpg more than you on the highway. My truck is a 2 wheel drive with 3.55 gears and a bed cover however.