My girlfriend has a 95 honda civic that was getting OK mileage when she first got it, about 330 a tank. Now it barely gets 220 to a tank. We changed the air filter and fuel filter and the oil. But still nothing helped... what else could it be?

This link should help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post293582

Installing new spark plugs (NGK only), if they have not been changed in the last two years, will likely give you the biggest bang for the buck.

I changed them when she got the car

Go over the primary and secondary ignition system. Ensure there is good spark at each plug and that there is not corrosion on the contacts inside the distributor cap, which can cause the igniter and rotor to fail. Ultimately, this can result in catalytic converter failure, due to unburned fuel. Finally, attach a timing light and check the timing.