Dramatic drop in gas mileage

I have a 91 4.0 4x4 Auto trans. It normally gets around 17 mpg. All of the sudden I noticed the fuel gauge dropping like crazy. Almost as dramatic as a leak, but I can find or smell none. I got 11 mpg last tank. Some black soot in the tailpipe. Any ideas? I am going with o2 sensor?

Have you checked for any codes? Clogged cat is a possibility also. Any other symptoms? Try new fuel source?

No CEL. Being a 91 I can't real time it with my code reader. It has been a couple tanks now. Couple different gas stations. People blame bad gas frequently. I have been driving automobiles and riding bikes for 2 decades and never got gas that wouldn't move my vehicle. Not saying it can't happen, but I am pretty sure that's not it.

It Does have a suspected head problem as it horribly fouls plugs 1 and 3 as seen from the front of the motor and exhaust gasses are present in the coolant. I run it till it misses then swap the plugs. About 6-8k miles. But it has been in that condition for the last 30k miles still returning a very consistent 16-17mpg. The plugs are currently clean of the normal buildup, just black and sooty.

OK, lets just say the O2 sensor is bad (which is legitimate). If you replace the O2 sensor, do you know how to verify the CAT is not plugged?

Point is: One thing leads to another, which is possible in your case.