Very Very POOR Gas Mileage?

For some reason I get the worst gas mileage I have ever seen. I finally checked it because I go through gas like beer when I am out and I am only getting like 30 miles to a quarter of a tank. The car has 54000 miles on it and I use synthetic regulary(last oil change I went 2500 over but this should not be the cause. I do have alot of mods which are

Hotshot Header
Hotshot Intake w/ K&N
Big Bore Throttle Body
Magnecore Wires
NGK plugs
Dynomax S/S Muffler

These I dont believe would affect anything:
Strutbrace
Rear Sway
Short Shifter

As far as power my low end kinda suxs and doesnt really move out until atleaset 3500 rpm. I am going to need tires soon but I want to wait until after winter . Also I have a little bit of a lead foot but I had tested this quarter tank and drove like any given 90 year old man. So I am not sure what it is. Maybe rings and seals, fuel filter, something loose or do these mods suck gas like this? I am not sure exactly what a fuel pressure regulator does (give the engine more gas or conserve it) but would this maybe help? Any help would be greatly appreciated. I will have to take out another job just for gas it I cannot solve this problem..Thanks for any help

Forgot! It is a 97 SC2 and smoke is pretty heavy out the muf. I wipe it clean and it is dirty 2-3 days later.

[DonfromNaples]

sounds more like a computer sensor than anything else. O2 sensors will create this as will ECT sensors(engine coolant temperature control)

[SATURN-V]

I suggest checking gas milage over 3 or 4 tankfuls, while driving easy. This
is the most accurate way to figure gas milage. I believe the gas gage is not
accurate enough, especially not between the quarter marks. (Next time you stop,
note what the gas gage is reading just before you shut off the engine, and
compare that to what it reads when you restart it again. This will give you an
idea how accurate the gas gage is. Or is not.)

The headers and low restriction muffler should INCREASE gas milage. Low
restriction intakes will lower gas milage a bit, but not down to 30 miles per
quarter tank. A performance fuel pressure regulator increases the pressure
behind the fuel injectors, making them spurt more fuel, hopefully just enough to
match the increased air flow at wide open throttle from the low restriction
intake. As such, a new regulator will lower milage a bit.

Black smoke coming out the tail pipe can be a sign of carbon build up in the
engine, or the engine is way out of tune, both of which can reduce gas milage.
White smoke that smells sickly sweet could be coolant that is leaking into the
combustion chamber; I guess enough of it might inhibit combustion enough to
lower milage. White or blue-ish white smoke that smells like exhaust is most
likely oil burning. I do not think that in and of itself will affect milage,
but a blown head gasket might lower compression enough to lower gas milage.
Watch your oil and coolant levels to see if they are going down. If not, the
quickest way to figure this out is have your local Saturn retailer put it on the
scan tool.

[L100]

I concur with the previous posts. In addition, I would also clean/replace your plugs when you are through with replacing the O2 sensor or the ECT sensor. The plugs are probably fouled.

[ssicarman]

If the smoke out the tail pipe is black and the engine runs rough (like a carbbed car with to much choke) then you likely have a bad PCM coolant temp sensor.

[SATURN-V]

Everyone please ignore the part in my above post about carbon in the engine; it
is incorrect.