13 mpg !!!!!!!!!!

Hi, I have a 1998 eclipse gs, and i am getting only about 13 miles per gallon. I have calculated it twice and it is around 13. I thought it might be the Oxygen Sensor, but wouldn't the check engine light come on? Please help me thanks.

The O2 sensor does fail on the low side, causin the ECU to run the engine richer than it should. And it can affect mileage before the ECU realizes whats happening (based on duel trims changes) and throws the code for it. But 13 is reeeeally low. There might be other issues as well.