Peaked out at 25 highway, in a level/gentle rolling-hills scenario. That was a couple years ago, beautiful weather that day.
Right now I have no problem hitting about 23 in a level/gentle hills scenario. Did it just a while back.
City just varies too much for me to get a locked-down answer on. My local roads are 15MPH speed limit, which is ridiculous. At the 25MPH limit of my old town I could get 14-16.
A steep grade, followed by a bunch of stupid turns and all ups-and-downs and all that... 14-15MPG. This is because the cruise control doesn't like the thought of doing 5 under for a minute, and will ramp it down to 2nd gear to climb the grade at 4k RPM. You can easily scavenge your mileage in an uphill scenario by finding the throttle position that would keep you at a certain speed level, and just holding it up the grade. If you slow down 5-10 MPH, big whoop, it's a lot better than downshifting. It might flip to 3rd, or just disengage the TC lockup-clutch, but anything helps. This metric was also taken on winter blend, usually it's a little over 15. This is at 45 MPH for the initial climb and a couple miles in, then 55, slow down to 35 for a sharp corner then get back up, go a bit more, slow down to 15 and get back up, final big hill climb, downhill to a 35 MPH zone, then getting onto another road, and doing 70 up a couple long hills. Mixed highway driving?
This is with the I6 and 3.73 gears, obviously 3.42 will give you some sort of benefit. Keeping up on various maintenance items will help a bundle, too. This includes drivetrain fluids, various filters, spark plugs, throttle-body cleaning, and various checks now and again.
You're likely to see a ton more different answers on the MPG question, but maintenance and driver-care are two big ones on maximizing the ratio. Don't let things slide, and make more time for travel, vs making up time by speeding/hard acceleration. A general rule of thumb which I figure is easier to do with the 3.42 is keeping the RPM under 2k while accelerating. If you turn of cruise, don't use it to re-accelerate back to the desired speed, it tends to just want to hurry up and get there (60-75% throttle sometimes it seems like). Get back up close to the speed, hit the resume and let it do the rest.