So you're the sausage KING of Chicago? :D
The gauge is just a gauge - not a meter. It's not calibrated at either end or in the middle. The way to check MPG is 1) fill up, 2) reset the trip odo, 3) drive, 4) fill up, 5) divide miles on the trip odo by gallons to refill. You don't have to drive to EMPTY, or even close - just enough to get a good average reading. But the closer you get to E, the more accurate your calculation will be.
Don't mess with the MAF unless you know there's a problem with it. It's a delicate device, easily damaged by improper cleaning.
If the plugs aren't due for changing, leave them alone. If you EVER install a plug in that engine, be sure to use a torque wrench or a torque-limiting device. It's easy to strip the threads, and expensive to fix them right.