drop in gas mileage

I have recently noticed a huge drop in gas mileage in my 2000, cavalier.

I went from getting roughly 320 miles on my 12 gallon tank to anout 260 on a 12 gallon tank. for those of you who dont care to do the math from about 27 MPG to 22MPG.

My question is what on earth could have caused this.

I will preface by saying that I am not the primary driver of the vehical any longer and it could be contributed to new owner (my boyfriends) driving habbits... but could it be that drastic... I was also under the assumption that it should be getting between 24 and 33 mpg.
I've also been told that it could be the 02 sensor, however there are no check engine lights on. And if that were the case that you pretty much have to replace both of them, front and back, because its hard to tell which one it is.
I god the car in 2002 with 70,000 miles on it.
It has about 130,000 miles on it currently.
I have recently replaced the spark plugs, changed the oil, air filter, put fuel injection cleaner in it, and have checked all the tire pressure and had the replaced not too long ago.

Any ideas?

Shot in the dark. Can you be the Primary driver for a week or so to see if it is indeed your boyfriends lead foot?
I too had a gas milage problem but found my tires were 4to5 lbs low in each tire. Once I brought them back up to 32to33lbs my gas milage increased by 4mpg.

possibly...

the only thing is that would mean he would have to drive my new car (an 06 Aveo), which he doesnt drive as soft as i would like him to. For at least the first few thousand miles i dont really want it to be driven as hard as he does also there could be problems with inssurance as well.

However, we were sharing the cavalier for a few months and this is when i noticed the mileage drop. I'm not sure if it had been happening before then, but i gauged it based on winter driving and last winter i got 320miles on a tank, this winter 260.
I'm not sure what its been averaging with him as primary driver since its nearly impossible to get him to run the trip gauge on a full tank of gas, or get him to understand how much better it is to fill up when you put gas inrather than just putting 5 or 10 bucks in at a time. Needless to say, even though hes the 'man' im responsible for making sure that the cars continue to run.

Here are few things you can do:
1. Change your fuel filter
- i changed my fuel filter and gain 6mpg
2. Use a good fuel injector cleaner
3. Set your tires to 30psi all around
4. Dont carry alot of stuff in the trunk.
5. Don't burn rubber or gun it
6. Clean out your throttle body.
- carbon build up can cause alot of problems
7. change air filter
- get a K&N air filter
8. make sure IAT is in the right spot