2.4 4cyl Within the last 2 yrs, fuel economy slowly decreasing from 32mpg hwy to 24mpg hwy with 21mpg combined. Within last 12 months, major tuneup (3rd since purchase) (plugs, oil&fuel filters), new O2 sensors and Passed SMOG no problem. Idles and runs smooth with plenty of power and acceleration. Oil & filter change every 3,000-4,000 miles since purchased car with 29,000 miles from dealer (lease trade-in). Need to add (1) quart every 1000 miles or so (since purchased).
This is where you will need a scanner capable of displaying live sensor data while driving. A good starting point is to view short and long-term fuel trims. If those numbers are high positive, the Engine Computer is requesting additional fuel beyond what was programmed in at the factory. If the numbers are high negative, the computer is trying to reduce the amount of fuel entering the engine and may be not having enough control to do so.
You can also watch the switching of the front oxygen sensor. If it tends to stay in the "lean" state longer than the "rich" state, you might look for an exhaust leak ahead of the catalytic converter. In between the pulses of exhaust gas pressure, the momentum creates a small pulse of vacuum that can draw in outside air. The oxygen will be detected as a lean condition and the computer will counter that by adding fuel. Oxygen sensors just detect oxygen, not unburned fuel, so no matter how much fuel the computer adds, the O2 sensor will still be reporting a lean condition.
The same thing happens if there is a misfire in any cylinder. You might smell the unburned fuel at the tail pipe but the O2 sensor will see the unburned oxygen.
Another less common problem that only applies to GM vehicles is mismatched fuel injectors. Chrysler buys their injectors from Bosch in flow-matched sets of four, six, eight, or ten and problems on those cars is unheard of. GM has a bin full of injectors of all different flow rates. They just grab a handful out of the bin as the engines come down the assembly line. Very often, after the mileage builds up on the engine and its characteristics change, some cylinders will be a little rich and some will be a little lean. Here again the computer can make up for the lean ones but it adds fuel to all cylinders equally. Now you have one, two, or three cylinders that were already too rich and you're adding more fuel to them. A lean cylinder often causes a misfire that you can feel, but when any cylinders are running a little rich, the unburned fuel just goes out the exhaust and is wasted. You usually won't feel that as a misfire.
There is a company in Indianapolis that rebuilds injectors and only sells them in flow-matched sets. GM owners are their biggest customers, and many people comment that their engines never ran so smoothly since they were new.