2001 Sunfire (2.2L)- TERRIBLE gas mileage

I have a 2001 Pontiac Sunfire with the 2.2L engine. Car has about 126,000 miles. I bought the car for cheap in a pinch until I got my other one fixed so I didn't know much about it. Anyways, this car is TERRIBLE on gas. I drive 160 miles round trip everyday, and had to fill the tank each day. I would have just under a quarter tank when I got home. So I figure about 215 miles to a full tank.

The original owner decided to remove the Catalytic Converter and cut the wire harness for the downstream O2 sensor. I know this will cut into fuel mileage but my mechanic over the phone said the downstream sensor will not make it do this. I can't get the car into him for a bit, so I thought Id ask here. I am replacing the coil packs, plugs, wires, and valve cover gasket tomorrow as its all original. Valve cover gasket also is leaking oil. Would all of that cause me to get THAT bad of mileage? Obviously it would affect it but from ~30mpg highway to 13-15mpg?

I am racking my brain on what could be causing this.... Fuel Pressure Regulator, Injectors, etc.?

Anybody else with the same problem or ideas to fix this? I don't really want to just throw parts at it looking for a fix. Thanks

That 02 sensor needs to be working. Is the inside of the tail pipe black. Does the vehicle run rough? I bet the check engine light is on, if so are there any codes?

The downstream O2 sensor is just for catalytic efficiency, has nothing to do with fuel mixture....

Check that fuel regulator vacuum line for fuel.....