96 Monte Carlo Z34 horrid gas mileage (13 mpg).

For the past month I have been getting about 13 mpg. I usually sit at 18-20 for a tank of gas.

I do not know too much about cars, but asking friends and then using the knowledge that I do have I have checked the following 4 things.

1) Cleaned my K&N air filter
2) Checked my tire pressure often
3) Checked spark plugs.
4) Oil change w/ new filter

Does the MAF affect mpg? What about the fuel injectors?

Any help would be appreciated, if you could also give me a basic idea of how to check something out without having to take it to the mechanic...I'd be even more grateful.

Thanks.