After having two, distinct commutes since the purchase of my Pacifica, I believe a LOT has to do with the factors of that actual commute itself.
When I first got my Pacifica, my company had not-yet moved to its current location, so for the first 3 months, it was a 53-mile commute one-way... 106 total daily.
It was about 90%-95% highway and I was doing (roughly) 3-day averages... resetting every 3-4 days... my COMBINED MPG for that commute was averaging between 29mpg and 31mpg... I was STUNNED.
Now... as of September... my commute is less-than HALF of that commute... I think it's about 23-miles one way... 46 total daily and about 85% highway. Oddly enough, my combined average MPG has dropped SIGNIFICANTLY with this new commute. I am averaging between 26-28 mpg... which isn't BAD by any means, but considerably LESS than my longer commute. One of the larger differences in the commutes is that the new one has substantially more "uneven" or "hilly" terrain. One might think that it all evens-out in the end: with every hill you climb, there's one to go down... especially since I average it over full commute trips (to and from)... so every hill you go up, you equally come down.
It's not a "traffic" thing as I travel on non-peak hours for both commutes. I actually think that there MIGHT be some credence to travel direction and headwinds... but cannot confirm. It SEEMED like my longer commute which was more of a North/South commute had cross-winds most of the time... but my NEW commute is a (mostly) East/West commute with areas of head/tail winds.
Tough to say... both highways are newly paved and smooth... neither commutes dealt with stop & go traffic... I wonder if it's simply a matter of the engine/drivetrain NOT being able to recoup the loss of energy from going uphil as well on the downhill parts... some efficiency is lost somehow. [shrug]
That's where I'M at so far.
"I'll procrastinate later..."