Tire Question? 195 vs 185 vs 175

The stock size is 175/65R15, isn't it?

 

Motegi Racing MR116 Black with Red Strip 15X6.5 I ordered these on Mon. so they should be here in a few days. I'm putting
Bridgestone Ecopia EP422 Plus 175/65R15 - 84H on them

 

Love the look of these! My wife's Prius C is Absolutely Red also, and these would look fantastic on it!

 

the issue is the weight of the tire and its diameter. go heavier and mpg will drop as well as acceleration if the tire has same diameter as OEM.
so if you go wider you need to reduce diameter by quarter or even half inch smaller. the advantage is better cornering and still get good mpg.
if you increase the diameter of your replacement tire you will see a drop in mpg and acceleration.
we have 205/50x15 tires on 15x7 wheels and exceed 50 mpg regularly on non contaminated gas without trying. with good hipo tires even corvettes are easy in the rain. dry if they don't rev to 3k before taking off. never bet against beating a model T across an intersection for the same reasons.
and a quarter-inch or half inch won't matter to your speedometer or odometer. its only a few % say 2 mph at 60 mph.

 

This site will answer almost any question you may have about tires, even if they spell them tyres.
Car Bibles : The Wheel and Tyre Bible

 

After reading Prius Chat and talking to two guys at Tire Rack I decided to go with the 185 instead of the 175 size tire.