After market rims..MPG?

If you stick with the same size, offset and weight then the difference mainly comes down to aerodynamics which will be small IF the wheels stay tucked in the wheel well. If you try to go for the flush look then the tire will intrude into the airflow and could increase frontal area (worse aerodynamics) if the wheel deflector cannot direct air around it properly. Toyota chose the wheel setup on our cars for a reason. The more we muck with the design the more likely we will decrease efficiency.

So, stick with a wheel that has the same size and offset as the stock wheel. Make sure the weight is similar or lighter. Use the OE tire or a more efficient LRR tire. This will ensure you minimize any potential mpg loss or even create a mpg gain.

Larger diameter wheels increase total weight and lower efficiency. It's been well documented. However, they look sweet and performance is much better as is safety but comfort is reduced. With larger wheels it is even more critical you follow the guidelines listed above if you want to minimize mpg loss.

The GenII suffered losses from 4-8mpg when going from OE 15s to 17s even with LRR tires and lightweight wheels. The GenIII seems to suffer a 2-6mpg loss depending on how much you utilize hypermiling techniques. Hypermilers tend to suffer more of a loss because 17" wheels/tires are harder to glide with. Stop and go urban driving will suffer worse than highway driving.

 

I just mounted 15x6.5 with stock rubber. Maxxim wheels about 13 lbs and it looks like I might be getting 1-2 MPG better.

 

The guy sent some hubcentric rings and I bought some locking lugs.

15X6.5 MAXXIM MAZE WHEEL 4X100/114.3MM ET38 RIM SILVER | eBay

 

The rings help center the wheel on the center hub, to balance the wheels on the rotor between the lugs. The wheels had a center bore of 73mm and the prius c is about 54mm, helps avoid vibration due to being off center. As for the tires, I've already put 2k miles on them and they ride well. No leak that I see and no problems.