According to fueleconomy.gov, the most fuel efficient minivan (2008 model) after the Mazda 5 is the
Honda Odyssey 6 cyl, 3.5 L, Automatic 5-spd, VCM, Regular (17 & 25). Whatever the VCM is makes a difference. The
Honda Odyssey 6 cyl, 3.5 L, Automatic 5-spd, Regular is much further down the list (16 & 23). In all honesty, though, except for the Mazda 5, it's all the same really. Check out
http://www.fueleconomy.gov/feg/byclass.htm and see for yourself.
It's too bad you can't get a manual transmission minivan in this county. In England, back in March, we rented a Kia Sedona minivan. It was a 5 speed diesel. Got great gas mileage and could seat 8. I really liked it. But DH, who works in the car leasing industry, says you can't get them in North America.
If you can find an older Chrysler Town & Country (2003 or older) or, more likely, a Chrysler Voyager, they used to come in a 4 cyl engine and were rated 18 & 25. Heck, I got close to that in my 1999 6 cyl T&C so you could likely do even better. They seat 7 and are CHEAP to maintain. I loved my T&C but gave it up when we decided to stop at two kids. I'm in a manual transmission 2006 Pontiac Vibe averaging 35/36 MPG. Worth the sacrifice but I still miss my luxurious T&C. Around here, you could get a 2002 Chrysler Voyager 4 cyl for about $8K it looks like.
Good luck. Car shopping can be so stressful.
--erin