Damn, back to this a year later.
You are greatly exaggerating the effects of tire width on MPG. Yes, theoretically you could get a few more mpg if you have tiny tires with less traction. No, it's not enough to make a difference. To date, I've had 3 different sized sets of wheels and tires on my car. I got almost exactly the same mileage with 225/40/18 as I do with 205/55/16's. I actually get a few mpg better with my 215/40/17's because those wheels are ~10 lbs lighter (each) than the other 2 sets I've used.
Putting wide tires on a hybrid makes no sense, they have no power and are not "performance" cars, so you wouldn't gain anything useful. The only cars that come from the factory with wide tires are "performance" cars with higher output engines that are less concerned with MPG and more concerned with driving characteristics. You could put wide tires on a Prius and it would still get good MPG. You could also put bicycle-width tires on a Viper and it would still get crappy MPG.
