Sudden plummet in FE and nothing seems to fix it
 

I have a 1996 Nissan Maxima and I have always got about 30 miles per gallon or more, but for some reason, here recently, my FE dropped for no obvious reason, and I am trying harder than ever! I replaced my O2's, I have checked my air filter (it is in brand new condition), I have changed my oil regularly (with synthetic), I have pumped my tires to 45 psi, and much more.

For some reason, now, I can not even get 20 mpg, but I used to always get at least 30

Nothing seems to fix this! Does anybody have any clue what could cause a sudden drop in FE this intense?

I'd look at the electrical components; Spark plugs, alternator belt, battery connections. weak spark doesn't burn the fuel as well.

Maybe a new muffler if you haven't had it replaced before.

Ah, a new muffler, huh? I did not think about that, as a matter of fact, my muffler has got quite loud lately, also, it is the original, never been replaced before, and has holes in it, so this just might be worth looking into. I still don't understand how that alone could cause a change this HUGE! I am getting about 15 mpg, but I used to get about 32! I am even trying harder now than I used to!

Do a compression check...Check Cat to see if plugged...Have you lost power?

if the cat is clogged it would be glowing red.

Well, yes, it does seem it has less power than it used to. When I first got it, you could easily spin the tires without even trying, but now, it seems that even at WOT from a complete stop, it will not spin. As far as the cat, I have been getting trouble codes on that for months now, but I have no funding for this, and my father refuses to pay for a new one when he can always reset the codes before inspection, however, if I had any income, I would definately do this.

might as well try the muffler: if that doesn't breath, neither does the cat.

Well, the muffler is definitely breathing just fine, it has got excessively loud, and because of my worn out rear suspension (rear sits excessively low) and the fact that (for some stupid reason) I tend to go "All-terrain" in it as if it were a jeep or something, there are many exhaust leaks in the muffler AND pre-muffler because they have both scraped a lot of rough surfaces (there are holes in both the muffler and pre-muffler).

off road cars? That brings back memories( kiss destroyer eight track days).
You may just be letting the car run more to keep warm since it's colder now.
New plugs usually give better mileage and don't cost much ( can always just clean and check gap).There are many ways to lose MPG; same amount to get it back.

we called it stump jumpin