I changed the air filter and checked the air in the tires, but I'm wondering if there's anything else that might be causing such a dramatically lower gas mileage?

I had the spark plugs and wires done a little less than a year ago. Would they need to be redone by now?

How could I forget an O2 sensor?? Of course! Likely suspect. Yes, check your O2!

I did some research on the O2 sensor and it seems that if it were the problem I would probably experience some performance problems, or even a check engine light. The car runs normal, no odd behavior at all, nor are there any warning lights.

I have a habit of only filling up the car about five or six gallons and running it on that until it's about empty. It helps me from driving unnecessarily. Someone said once that doing that can cause the fuel injector to get dirty faster.

Is that true? And if so, could that be my problem?

The O2 sensor can be bad, but not bad enough to trigger a check engine light.