Rims

Lets apply some logic, to answer your question.

1a) People go UP in rim size = more weight b/c its more material to make the larger wheel.
1b) Its possible one could go DOWN in rim size, but I have only seen this done w/ low riders. Maybe you will be the first low riding Prius w/ hydraulics?

2) A larger tire is required to fit the larger wheel; diameter or diameter and width. This means more weight b/c more material is required to make the larger tire.

3) Larger wheel + larger tire = more rotational weight = more energy expended = lower MPG compared to the stock wheel & tire.

4) EXCEPTION. If the larger wheel was made of some ultra light material, say carbon fiber or titanium, then you are talking about a significant weight REDUCTION compared to the stock wheel. You will likely see an increase in MPG. However, such a wheel would be ridiculously expensive, if not already stolen.

5) If you are contemplating wheel/tire upgrades, you would want to strive for minimal overall weight gain compared to the stock wheel & tire.

 

I opted to upgrade to 18x7.5 forged wheel which just a pound lighter than the original 15's the car came with. I tried to keep the tire size the same diameter as the stocks to not affect the mileage too much. Since install I've seen a little decrease in MPG. First week with the wheels I noticed 5 MPG loss.
As of now i'm 8-10 mpg down BUT I have low tire pressure which could have had some play in the extra loss of mileage. I'll be adding air to the tires today and see if there's any changes.

 

I didn't expect THAT much of a loss either but I think once the tire pressure is back to where it needs to be the mileage should kick back up to the 60's
I commute 70 miles a day for work so I want to get as much MPG as I can while still upgrading and making the car look the way I want.