Here are a few tips, things that you can do.
First and for most forget the built in fuel guesstimator, for that it all it is, a guesstimator. They only way you get accurate milage is to put pencil to paper.
Second forget about the higher performance vehicles you have driven, the Passport isn't a high performance vehicle, It's a 4000 lb + vehicle being powered by a 280 hp 6 cylinder engine designed to go from A to B over highways, city streets and light to medium off road situations. It will drive differently.
Third, keep in mind that small city commutes are a gas mileage killer. Every stop light you hit, every stop sign etc takes the real time gas milage down to Zero. Zero is an average killer cause if you're not moving you not racking up mileage.
Things to do:
When you take off from a stop do so slowly and smoothly. Quick acceleration is a gas guzzler.
Drive the speed limit and keep you speed at the speed limit. If that means driving in the right hand lane so be it. When you are exceeding the speed limit no matter what the lane you will invariably come upon slower traffic that cause you to slow down, change lanes, speed past them, weave in and out of traffic all of which will effect you gas mileage. Learn to drive like you were driving a Limo with a very important client in the back. They want a smooth comfortable quiet ride.
Use your brakes sparingly. Let the vehicle slow down naturally when approaching a stop light or stop sign, slower traffic etc, which for many is tough to do as it means leaving proper space between yourself and the vehicle in front of you. This is done to a point. You don't need to let off the gas two blocks from a light and travel the last half block at 2 mph, but neither do you want to zoom up, slam on the brakes and then take off.
Learn to drive more than 10 feet in front of the hood. This will indirectly effect your mileage as you will be able to prepare for what ever is coming allowing you to drive smoothly as you continue your route. You need to know what color the light is not just the one you are coming to, but the next one and the one after that, as far as you can see.
Put down the phone, learn to talk without your hands, if you are hand talker, don't keep fiddling with the radio etc. Again an indirect effect on gas mileage. If you ever watch a texting driver, one who has to look at the passenger to talk, or hand talk with a passenger(s) you will see that they slow down, speed up, slow down, weave around, etc. Again smooth steady driving is what you want.
Finally, if you want to know what your real MPG is go out an test it properly. Find a nice state highway, instead of an interstate, fill the vehicle at a station close to the highway and drive as described as above. Drive 30 minutes or so at the speed limit, turn around an return by the same route, then stop at the same gas station and fill the vehicle up exactly as you did before the drive. Put pen to paper and see what the real MPG was. You can do the same thing on a nice slow county road to get an idea of what you MPG is at city speeds as well.
FYI, I'm averaging between 19 -21 mpg in town and 26 -28.8 on the highway depending on whether I am driving on a flat highway or going through hill country.