Nissan Repair: Gas mileage plummeted, 2004 infiniti g35x, gas mileage

Question
QUESTION: I am the original owner of a well-maintained 2004 Infiniti G35X Sedan with 205,000 miles on it.  I have no performance issues and no error codes on the dash (they do all light up when I start it and then they go out).  The RPMS return to their normal position once it is warmed up, after shifting, when idling, and coasting.  I can't see that a thermostate is sticking.  I do have an issue where spark plug 4 fouls on me and has to be watched, but this usually causes the car to run rough when it needs to be replaced and this is not happening presently.  The car usually gets 20 MPG and has suddenly dropped to 12 MPG.  It is not in AWD presently and it is obvious when it is.  The car does consume oil and I am dilligent about staying on top of this and getting regular oil changes.  The air filter is an Airhog and is clean.  WHAT could be the problem?  Please help ... frustrated to no end.  The car is in St. Louis, MO if it matters.

ANSWER: Hi Lisa -

Have you checked your transmission for slipping?

That is a tremendous difference in gas mileage. Any indication of loss of power, or fuel line leakage?

Aloha
Calvin

---------- FOLLOW-UP ----------

QUESTION: There doesn't seem to be a problem w/the transmission, no loss of power, no fuel leaking.  The car is performing well?  I filled it and put in on the highway and got almost 20 mpg back.  The latest gas has been a combination of city and highway and it came back at 16.

Answer
Hi Lisa -

Well. The last numbers you have given me are more or less back to normal, much different from the 12MPG you stated originally.

I assume you have a V6 engine so your 20/16 mileage is OK for the mileage and year of your vehicle.

aloha
Calvin