Why is my car burning so much gas?

I have a 93 Honda Accord.. and it usually gets 23 miles/gallon..
but the past couple of weeks it's been burning gas like no tomorrow.

I just filled it halfway last night, and only went to a few places (about 15-20 miles worth of driving), and now...

it's below a quarter..

Some people are saying I need a tune up.. others are saying it's my air filter, but I just got a new one a couple of months ago and it's still clean.

What is it?
Thanks

Is your check engine light on? If so take it to an auto parts store and most will test it either for free, or like $20.00 with their little code reader and tell you whats wrong with your car. You then can work from there.

There are just so many things that can cause you to get worse gas millage its hard to say.

Nah, the check engine light isn't on... I have my own diagnostic tool, but yeah.. it won't read anything.. I know I need a tune up, but I was wondering if that was the cause of it.
Thank you!