Are you absolutely sure you have an 18 gallon tank? Many later SHOs got the standard 15 gallon instead; my 93 has a 15 gallon tank but my 91 has an 18 gallon. Come to think of it, every gen II I've owned save for SinisterSHO's black 93 has had a 15 gallon tank. That's a 95 ATX, 95 MTX and my current 93 ATX.
If you do have the big tank, I'm going to assume by your post that you don't drive it too much, since you're just now finishing your first tank after nearly a month of having the car. If you typically drive very short distances the car will never have a chance to get up to normal operating temperature; as such it will run much richer because it's still cold, which is exactly what it's supposed to do. The downside of course is reduced fuel mileage. I'm seeing a slight reduction in fuel mileage on my 93 because of this. My wife used to drive it 18 miles one-way to work, but for the last couple months it's only been about 6 miles. She was getting 23-24 miles per gallon, but now it's down to about 20 because most of the miles being put on the car are now done so while the engine is cold. She also doesn't hit the secondaries too often, if she did I would expect to see mileage in the 16-18 range. 18 MPG is the EPA city rating for an automatic, so 16--while not great--really isn't too far off from the expected city mileage.
