JamesinLV is absolutely correct. The fuel warning light comes on at 1/4 tank. I recently took a 130 mile round trip and I averaged 17 mpg mixed city/hwy (mostly highway). I think the average is more like 11 mpg in city, 18-20 on open road, primarily in P400 D3, D4 which short shifts and gets you to gear 7 rather quickly. If you spend any time in S4 or higher and drive aggressively, you will drink gas like its going out of style!

 

I have almost 24,000 miles on the car, and I note the mileage at every fillup.

15.1,14.7,11.8, 12.2,13.6,14.7,17.1,15.6,12.9,12.5,11.5,11.3 Average of 13.6 mpg SD of 1.8

This covers mileage from 21,600 to 23,800. Interestingly, this also covered my mileage since I switched to using only 94 octane gas. Here are the mileages before, using 93 octane, covering 19,600 to 21,600.

11.6,12.0,11.3,13.8,15.2,10.8,9.5,10.9,10.6,12.5,1 6.1,10.6 Average of 12.1 mpg SD of 2.0

Not scientific, since I think there were a few more hwy miles in the 94 octane group. But still pretty interesting. Based on the low mileage points, it looks like the mileage is better with the 94 octane.

 

Quick update. Just took my car for it's first longer trip...from Vegas to Riverside, CA for about 250 miles. Average of 19.7mpg, which included lots of time at 90-95mph when there wasn't traffic holding me back to about 60-70mph. Without a lot of 20mph speed differences because of traffic, I have no doubt I could have pushed it above 20mpg especially since the net elevation difference is also about -2000 feet from my home down to Riverside.