Can fuel mileage be affected this badly by a bad catalytic converter?

What is your question?

So I have a 2007 Honda accord 5 speed 2.4L 4cyl and I've had issues with a p0420 code for a couple months now where it comes on and goes away every few days and I planned to eventually replace the cat because I know it is for sure the cat and not the sensor. However today I noticed something, after I filled up from a little under a 1/4 tank to full which was about 12.5 gallons or so I drove 180 miles round trip cruising at about 70-90mph both ways. After I got home I noticed that my tank was again below about 1/4 tank which didn't sit with me right. Can this all be just because of a bad catalytic converter or is it low enough mpg to the point where it seems like something else is going on?