Plunging fuel economy?

[TMACK]

OK, forum, help me out here.
I'm comfortably cruising along these past two fill ups, pegging a respectful increase of 47 mpg, thinking it is a combination of the warmer temperatures, possibly the gas stations changing to their *summer* formulation and my changing to manual shifting at lower rpm.

Life was great, I was on my way to the elusive (to me) 400 mile tank range, when WHAM: I notice my fuel gage is depleting faster than normal, even though the SGII is reporting 44-47 mpg each trip. My last fillup gave me a 30 mpg!

What can explain this? My driving technique did not change, nor did my daily route. My tires are +3 psi over the recommended, my oil and filters are new, I have not increased my weight loads. The only out-of-the-ordinary thing I have done this 100 mile run is intermittently engage the A/C for cabin window defogging during the recent rains. Could this possibly be the reason for such a dismal economy?

[JPaul]

Unusual – freak? – previous to last fill up: average the last two tanks and you get... what? About 39 MPG. That could be it? My ¢2.

[TMACK]

Freaky indeed. I am wondering if I should engage the A/C more: in my quest for fuel economy, I rarely use it; only when the wife is with me and smacks me down.

That is: perhaps my compressor has not *broken in" with respect to the rest of the vehicle?

[jetfuel]

..A/C is way up there with driving habits as the big mpg killers..
..but...just in case...have you taking a look at your plugs?...and please don't tell me that they are XXX miles old...they can/will fail at any time...

jetfuel...just me...

[TMACK]

Come on, jet: the thing is only at 11K.
I guess I could go out and check when I find a spare 2 hours, but would I not notice other symptoms for bad plugs, like poor response or slugishness?

The Iridiums are on my to-do list, however; but just as an excessive luxury and possible economy gain.

[nonuro]

and check your tires

[TMACK]

Read my post, mate: I'm, well, OCD about tire pressure.
Indeed, I check twice a week.

[jetfuel]

...I know where you're coming from TMAK..but...a little corrosion on those plug threads and the firing voltage change will not always give you that poor/sluggish feeling...

jetfuel...totally grounded

[gabe1475]

If I had to guess on your last fillup (early fuel shutoff) resulting in higher MPG numbers, and on current fillup you probably filled up properly. The only question is did you adjust the scangauge last fillup, if you did that would adjust the MPG down on the current fillup from the scangauge estimates.

[rfernatt]

Two words: pump shutoff. There is too much variation in a single fill to make any meaningful conclusions. I've had great 52 mpg tanks (early pump shutoff) and not-so-good tanks in the 30s (late pump shutoff). You average it all together and get something somewhat reasonable. Here's a breakdown of my 2009 mileage by season:

Season. Avg. Low. Hi
Dec-Feb 41.1 33.9 43.6
Mar-May 42.9 38.2 48.3
Jun-Aug 44.5 41.0 48.7
Sep-Nov 44.3 41.2 49.7