Metallicat, I don't know where Northville is... I lived in SE Michigan for 20 years. The expressways around here in Florida and flat as a pancake and smooth. I know most of the roads in Michigan are tore up from the big trucks.
I believe that the road surface could play a part in that. I know when I am coasting [off the gas] my car will go forever without slowing drastically. On a rougher surface road, it will slow quicker. I figure that's more rolling resistance, and that could factor into the mileage?
Are you using cruise control? I am an avid fan of that, I I think that it yields better mileage, since I know some poeple "pulsate" the gas pedal with their foot, on and off, on and off. You may not feel it, but the car will use more gas that way. I use cruise anytime I can.
From the best I remember, I can get the car to shift into 6th gear [automatic/V6] around 42mph. There, it's at about 1,200rpm. 50mph is exactly 1,500rpm.
I believe that 70mph is 2,400rpm, maybe? I'll check on the way home from work for ya and post back. What kind of rpm's are you turning in the 4-banger?
Also, I noticed a pretty decent increase with the Jusnes tune, when I keep my foot out of it, at least. You?