Get some way to see instant FE -- if the SG-2 is too expensive, get something to show your air/fuel ratio versus how hard you're pressing the gas pedal (seat-of pants maybe) and you have a cheapo version. Otherwise, 99% of us here swear by the ScanGauge. The other 1% owns SUVs, so they left
As mentioned, over-inflate your tires, slow down (keep it under 60 mph if possible), coast as much as possible and...
i had a 96 plymouth breeze 2.0 L engine with manual trans, i routinely got 23-27 mpg mixed driving.
as far as i am concerned, it got rotten mileage for the size of the engine even though i wasnt' trying to get good fe.
i think the caliber is a pretty practical vehicle, it's a small car that you can pack a tonne of crap in it and it isn't an suv.
you can probably do without the 2.4 L engine, but i think chrysler got it right with the caliber.