That's incredible. Best I've ever done is 19. And that was really trying.
Typical is 13-14, as it spends most of its time around town.

I usually get around 22-23 highway milage in my 2005 4.4.

Just did a run from Colorado to California and back and averaged 26.1 averaging almost 80 on the highway and of course less in the city. Over 20k on it and still enjoying it.

Out of curiousity, I wonder if there is a slight decrease in MPG on the 20" tires. You would think it has some adverse effects on MPG vs. the stock 19"s since they have so much more rubber contact.on the ground.

same situation here and we are getting roughly around 21mpg

They actually have the exact same rubber contact on the ground, just with a different shape contact patch.

As long as the two tires being compared have the same rolling circumference, meaning the odometer and speedometer are still accurate, then wider tires will have a negative impact due to increased frontal area, thus more air resistance. Not a big issue, though.

Many tires will have a different rolling resistance, in terms of efficiency, and it doesn't depend on width. If you think about the squirming that happens as the contact patch deforms, with each revolution, that takes energy, and thus causes an increased rolling resistance. The stiffness of the tire structurally plays into this, as well as the rubber compound. You can have narrow or wide tires with higher or lower rolling resistance.

We just picked up a super clean (Kansas car) CPO'd 4.8i last weekend and haven't left the city, but OBC is reading 16 mpg. All options minus DVD and pricepoint was at least $15K less than a comparably equipped 35d. so I paid the 15k to myself as a non-eco credit and will enjoy the V8 without too much guilt or worry. Curious to see highway mileage on a trip.

Makes sense.

Just picked-up a 2011 x35d last weekend and got 26.0 MPG average on the way home (85 miles going through Orange County)! Needless to say, I'm stoked!

Concur...
 

19.4 highway is the best I've done with my 4.8i...