Any Gen 3 w/ 205/60/15's?

IMO, the only thing that tire will do is make your MPG a little lower. It may also affect the readout of the speedometer and odometer.

Study the information found here for a good explanation of tire sizes and how they work.

Tire Tech Information - Tire Size Guide

FWIW, I am not running the size you are asking about. Our Prius came with 17" tires and rims. IIRC, the correct size for your Gen III Prius should be 195/65R15

On a different note, what kind of MPG are getting with your short trips if you don't mind me asking?

 

Your mileage sounds about right to me. Once your Prius gets broken in, it should get even better mileage. A lot of it depends on how you drive it and also how you maintain it.

You will find that like other vehicles, the Prius will do worse during the winter and better during the warmer parts of the year. If you can drive when you don't have to use the A/C, your MPG will be even better. If you really want to get excellent MPG, practice what is known as Pulse and Glide when you can. Just don't hold the traffic up, otherwise someone might have a case of road rage at you.

I made a trip to the store tonight. Left with a cold ICE. Trip length was about 12 miles each way. Going to store averaged a little over 50, coming back average a little over 70. No A/C, ran with driver's window and passenger rear window down for cross ventilation.

This Prius I drive could do better if it had 15" rims on it, but alas it came with the flashy 17" rims. Don't forget when you do buy new tires, to look for LRR type tires. It does make a BIG difference.

 

You might be interested in these tires. They are recommended here on Prius Chat and quite a few members seem to be happy with them. The EP20 you have is the OEM tire. Not the best, but certainly not the worst either.

Bridgestone Ecopia EP422 Plus (H- or V-Speed Rated)