95 240sx runs well but gets bad mileage

Leaking fuel injectors?

mechanic says no leaking injectors.

Possible catalytic converter is starting to fail. You replaced the O2 sensor, was it throwing a code? That same code pertaining to the sensor could also pertain to a cat going bad. Bad cat = bad gas mileage. Eventually it will lead to rough running engine, frequent stalling. A shop can use a vacuum gauge to determine if there is any clogging in the cat.

when you're drifting that thing around Laguna Seca, you might use more gas-