You are going to need to do very specific MPG math figures to be sure of your milage, its funny too, when you know that you are testing for fuel economy, you do tend to drive better.

Call around for a shop with a 5 gas analyzer and ask them to just do a simple analysis for a quick check on your fully warmed up engine.

Step on the throttle semi hard and see how much black smoke comes out the tail pipe, much black smoke shows an extra bit of fuel being delivered. I would expect smoke on heavy accel but not gobbs of blackness.

You may want to opt for a scan data readout of the MAP and the O2 sensors (everything) along with a 5 gas readout???? One more thing, if the thermostat is stuck open and the computer keeps the engine in open loop then you will burn more fuel too but I would think (maybe, maybe not) this would set the CEL, many issues will not.

As mentioned, does your area have a change in their fuel blends.