Sudden Drop in MPG

[numero9]

What can cause a sudden drop in MPG? I used to average around 36mpg and have to fill up after 315 miles or so. Over my past two trips, my tank empties at 225 miles and only gets a calculated 27mpg. The routes have all been the same and nothing seems to be wrong or different with the car when running it.

The only difference is maybe it's been getting a little colder around here but not by much. I put new plugs, wires/dist head, new ATF fluid, and oil change about a month or so ago -- nothing wrong there. Any ideas of where to look?

I'm going to change the fuel filter soon since it's been overdue, but I'm really surprised that I'm losing nearly 10 mpg pretty much overnight especially with the car running just like it always has and no difference in driving.

Edit: Oh yeah -- 1990 Prizm base, auto, 4AFE

[eage8]

I would guess it's probably your o2 sensor...

or you used oil that was heavier.

[Tech1984]

The recent little spike in gas prices has to do with the switch over to winter blend fuels which cost more to produce.
Once you start filling up with this fuel, there's always a noticeable difference in mpg that you had all summer.

[numero9]

Oil weight has been the same as its always been, same brand. The car was getting 37 or still with the current oil.

10 mpg seems like a huge drop for a change in formulations...

[65Pontiac]

is it running rich?

[numero9]

Doesn't seem to be, I can't smell it or anything. The idle seems the same as always, too.

[90 GP]

I agree on the winter blend gas as the culprit. I have noticed my mileage has dropped as well but my highest never was anywhere close to your 37.

[numero9]

Getting a little bit better this time, but still needing to fill up around 275. :-\