The big OE spark plug makers don't even claim any MPG gains from changing to this plug or that. If they could, I got to believe they would. The different 'grades' of spark plug are mostly about how long they last. Copper plugs live the shortest, platinum is next, followed by iridium which I've found will go over 100k miles typically.
There are a few specialty plugs out there like Pulstar. I tried these about 10 years ago. They sent me a free set for evaluation. I keep track of my gas mileage a lot closer than most, and I didn't notice any difference. I ran those plugs for about a year until one died on me. I then swapped in my old plugs, and again I noticed no difference at all.
Personally, I typically stick with NGK or Denso plugs unless I find something else cheap.
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View my fuel log 2015 Mirage DE 1.2 manual: 47.2 mpg (US) ... 20.1 km/L ... 5.0 L/100 km ... 56.7 mpg (Imp)