The brand / model of the tires can make a big difference, as well. For example, some of the 1st gen Insight owners report a loss of 8-10 mpg when switching to a different tire (from stock).
I don't know offhand what tires the Prius 2 came with from the factory. But my car has the Firestone Fuel Fighter tires, which have a reputation for good fuel economy. They're also kinda pricey. So I'm glad the previous owner bought them.
Also, I'm using a pretty mild tire pressure increase. Stock tire pressures are 35 psi front / 32 psi rear. I increased it by 3. So now it's 38 psi front / 35 psi rear. I didn't want to use really high tire pressures and prematurely wear my not-so-cheap tires.