Real world fuel economy

One of the reasons we got our Mazda5 was because it was relatively fuel-efficient, especially compared to other minivans. The EPA rates the Mazda5 at 21/27, which is 13% better than the 2009 Honda Odyssey with cylinder deactivation, and 17% better than the 2009 Toyota Sienna. Of course, real world fuel economy is notoriously variable, and so it's important to examine real world data as well. With that in mind, here's our data from this trip. Overall, we drove 1916 miles and averaged 24.6 MPG. Most of these miles were driven with a 16 cubic foot cargo box attached to the top of the car, which adds a lot of drag and noticeably lowers fuel economy. We also traveled faster than the EPA fuel economy tests - on the open roads of Utah and Nevada, the cruise control was set at 80 mph, which lowers observed fuel economy further. I'm very happy to pay a few more dollars to get home a few hours earlier, especially with three small children in the car... =)

We attribute the two low mileage legs of our journey to some very heavy winds during a large storm in Nevada - with a big box on the roof, the car had to fight hard to maintain 80 mph despite all the buffeting.
The two highest mileage legs of the journey had quite a few trips around the Salt Lake area, without the box on the roof (since it could stay at our in-laws' house), and driving at 70 mph instead of 80. So it looks like the 27 MPG highway rating is fairly realistic.