When I bought my Subaru new, I think they said it got about 27 mpg on the highway, and 22 in the city. I had heard that these estimates are usually a little high, and this matches the fact that on most trips I only was seeing 24-25 mpg (though I was usually getting 22-23 around town).
But last winter, we did a road trip to Albuquerque. With the higher gas prices, I went through the car and did all the little recommendations to improve performance (adjust tire pressures, etc.). I also took the Yakima roof rack off. We seemed to have much better mileage on that trip than I was used to.
So for this trip, I decided to track the mileage. I found a way to stuff both bikes, 2 kayaks, and all our gear for 2 weeks inside the car (with just enough space to see out the rear view mirror), so again removed the roof rack. The vast majority of this was highway miles, though there was a significant amount of time in the parks at much slower speeds (including stop and go). We did have a day of driving around Yellowstone with the kayaks strapped to the roof, and a few days around Ketchum with the Yakima rack back on so we could carry bikes). The weather was warm, so we had the air conditioning for just about the entire 2 weeks. And we may have, um, exceeded the speed limit from time to time.
The results were surprising. Over 3578 miles, we averages 28.25 mpg! That's 5758 km and 8.3 liters per 100 km for those who prefer metric. Wow, my little car is not as fuel inefficient as I thought.