I'd like to change my Energi factory wheels to a set of 16x6.5 wheels. ...
Factory C-MAX wheels are +55 offset and 63.4mm/63.5mm center bore and 5x108mm bolt pattern. ...
Good set of specs; not everyone gets the center bore, and it's the most important. I avoid wheels that require a spacer to achieve the correct center bore, as spacers get lost, and without them, the wheel will never balance. I like the idea of finding Ford wheels for that reason.
Be aware that offset is a function of wheel width. That's +55 from the center plane of a 7" wheel. On a 6.5" wheel, a +55 offset will pull the wheel in 1/4" from stock, while giving 1/4" more clearance to the strut.
- 205/60/16 tires are WAY cheaper and more common, and they are the same size for odometer/gear ratio
- The new Nexen N Blue EV tires are only available in this size, and they are supposed to be the best for ULRR (and they're like 1/2 the price of comparable 225/50/17 tires)
- Overall better and cheaper tire availability
- Less rolling resistance and drag due to smaller (thinner) tire
- Slightly better ride quality, NVH and vehicle wear due to a larger sidewall tire...
...I understand the downsides of a thinner tire. I don't carry much weight and I'll be doing weight reduction on my Energi so it's a non-issue for me....
This is where you're missing a few things. If you like the Nexen N Blue's, use the 215/55-17 on stock rims for a best-fit option. Exactly the same 93V rating as OEM. I'll grant you don't need a V-speed rating, but you do need the load capacity.
The 205/60-16 is a 91H rating, and 77lb. lower load rating. It would be a very bad idea to reduce the tires' load capacity 300 lb. in the variant with a weight problem. This leads to things like loss of insurance coverage when you have an accident. What if your insurance company finds out you ran undersize tires and had a blowout? What if the guy you hit finds out, or his lawyer.... This is a real "better safe than sorry" issue.
That assumes that anyone would sell them to you. Tire Rack will not ship this tire for this car. You'll have to mislead them.
Or find a dealer that carries the right size.... It would be very interesting to hear from users of other tires. Just find one that fits; Tire Rack is servicing the Kia EV OEM demand with the 205/60.
I think the stock 225 width LRR tires already lack adequate grip especially under emergency braking. Going with an even lower RR tires and width decrease may make matter worse.
Then you want a narrower tire.
For a given tire pressure and axel load, the contact patch of a tire is constant. It's due to Pascal's Law, which states that:
force = pressure x area
A 185/65 will contact the same area as a 235/50, but one patch will be 6.2" wide, the other 8.2" wide (assuming the full tread width is in contact). Since the contact area is the same, the wider tire can only be in contact with the road over 3/4 the length of the narrower tire.
And the last bit is that tire traction is greatest in the direction with the greatest tread contact patch length. A long, skinny tire won't corner worth a dang, but it will start and stop extremely well. It's why "narrower is better" for snow tires; better traction in the "most important" direction, and less snow to push aside in front of the tire.