The Mazda5 gets better gas mileage than larger minivans, but in terms of highway mileage it's no standout.
With its new six-speed automatic, the Honda Odyssey can match the Mazda 5's EPA highway rating of 28 mpg; but the Honda can't quite equal the Mazda's city ratings of 21-22 mpg. Meanwhile, the Odyssey can carry eight passengers--two more than the Mazda5.
There are also a number of smaller-car alternatives to the Mazda 5 that get significantly better mileage; and the new Ford C-Max returns 43 mpg on the EPA cycle, from its hybrid powertrain.
But it's worth noting that we’ve seen significantly better real-world results in the Mazda. In one 420-mile, varied weekend drive of a manual-gearbox model, over two mountain passes, mostly highway driving plus some city miles, we averaged nearly 30 miles per gallon.