I got one last week, used @ 43k miles. I too was getting mid 30 mpg no matter what. After adding marvel mystery oil in the gas and oil (and changing oil) I didn't see an improvement. But I slowly pushed the gas pedal to the floor a couple times and kept it like that til I was doing 80+ from 40mph. Now it's much better. I assume it was driven very nice and never went into higher rpm's. Stuff will gum up over time and if you don't get rpm's high it will never get the sludge off. Of course you will probably need a system cleaner of some sort in the system. I thought there was something wrong but I think it was just never opened up on the highway.
FWIW, the "normal operating temperatures" given to cars is there for a reason. At ambient temps, the pistons are slightly smaller, then when the car is warmed, the pistons swell ever so slightly, thus meeting the intended tolerances for the engine. Until it's warmed up the piston has very slight lateral movement at a microscopic level. This is why engines experience the most wear at startup, a piston will "rattle" in the cylinder vs sliding. It's best to warm it up, and as the manual states, when the green temp icon disappears, it's warmed to operating temperature.
Physics and all..