I've had my 5/11 build Hemi Summit for one week and only have 400 miles on it. My commute is 4 miles of city/country driving to go to daycare then another 34 miles of highway driving with the flow of traffic at 60-80mph to get to work. Total round trip is 76 miles. I have yet to use the cruise control and commute with my wife so that's extra weight in the car.... but not much
. The tires are at 36psi as recommended and I'm in the auto mode. The dealer filled it up with 87 octane and I got 19.4mpg for the first 228 miles, 3 days of commuting. With half a tank I put in 93 to get around a 90 octane mix and reset the mileage. The first day driving to work my mileage jumped up to 20.9 as has stayed there. It's not a long term test and the engine is not broken in yet but I've seen a little increase in mpg using 89 octane that more than offsets the extra cost.
I am satisfied but I am hoping to get just a little more once broken in. On my Mercedes GL550 the rating was 13/17 and I averaged over 20mpg going back and forth to work so I was hoping with 13/19 I would get closer to 22.
I have a K&N drop in filter getting delivered tomorrow and I've always seen a 5-10% increase in mileage with them on previous cars. In my experience the smaller the factory airbox the bigger difference it makes. The Mercedes used two filters with a total surface area of 142 square inches. The Jeep's filter is 85 square inches on a motor with similar ratings.
I'll keep posting my results.