2011 tires

from my understanding, bigger tires are better in the summer, (bigger circumference would achieve you more mpg because the car you move more on one rotation)

but,

in the winter, smaller tires are better (not as wide, it will go through snow easier than having a very wide tire)

you also have to check how much changing size on a car after you bought it can affect your speedometer... if the car is rated for 14" tires, and you decide to put 17" because you can/want, well your speed display will be wrong... (which if you drive always to the limit could result in getting a speeding ticket) i am not sure of how much it affects but it's something to think about

 

You need to maintain close to the same circumference (revolutions per mile) because the gear ratios and the drive train are the same in cars with either size tire. So to replace the 195/65-15 with 16s you might want to choose 205/55-16 for example.

The 15 and 17 tires presently used on the Prius have approximately the same circumference, actually the 15 inch are slightly taller.The 215s are of course 20mm wider.

The 215 size will afford better traction for cornering and braking under virtually all conditions except snow. The narrower tires are better in the snow. The 195 size will provide a little better mileage, my estimate about 2 mpg under most conditions. The 195s may also provide a slightly better ride and might last a little longer, although that still remains to be seen.

 

This is not that simple of a change as swapping the tires and rims. There are some mechanical differences in the settings for these two vehicles, and that must be taken into account. Also, in addition to the tires, and the traction, you will be increasing the unsprung wieght dramatically, thus affecting your mpg by 2-5%, per many folks here, and my own personal observations.

Read the forum for more details on this, or you could have a problem on your hands.