Rims

Depends on a lot of factors. What's the weight of the wheels, what's the width of the wheels? Are you using the same tires? What's the offset of the rim?
If they're heavier, you're putting more resistance on the system, dropping mpgs.
If the tire is wider, you're potentially creating a larger contact patch, dropping mpgs.
If the offset is more on the new wheel, you could be putting the wheel outside of the aero of the fender, dropping mpgs.
If you use a different tire, make sure to use another LRR tire so that you don't drop mpgs.

 

I don't know what the right specs should be. I just know what to look out for. I do know there are a few threads you could search for these answers, though.
Sorry.

The fuel max tires will be fine. They are LRR tires and shouldn't change your mpg's much, if any.

 

If those rims fit your car (and they should, with dual drilling), they'll be fine. Not too heavy at 16 lbs and the offset isn't too far off factory. The tires look like a good choice as well.

 

Tires, I don't know, but the sepcs on those wheels are very close to Konig Helium

https://www.americastire.com/buy-wheels/konig-helium

which many users on this forum, including me, have put on the C with no negative impact, so you should be OK. There was much discussion about these and other wheels a few years back.

You will also need conical lug nuts. The acorn-shaped OEMs will damage the soft aluminum wheel.

 

If the weight listed for those is correct (11 lbs), that would be a lot better than the 16 lb wheels linked previously.

 

It is accurate. They're not forged, and not the absolute lightest 15" wheels out there, but good value nonetheless. When I first got my C, I sold my OEM wheels and hubcaps on CL, so net out-of-pocket cost to me was around $100, LOL.

 

Wonder why Discount Tire (from that link) tells me that the Konig Heliums don't fit my 2015 Prius C when I've seen so many on this forum use them.