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Spark plugs have next to no effect on fuel economy as long as they're working. The different types of spark plugs simply determine how long they last. Coppers go ~15k miles (all numbers are YMMV, also you can regap them to go longer). Platinums are more around 50k miles, and irridiums are good for 100k miles.
Personally, I think a lot of people have had issues with platinum plugs, so I tend to stay away from them. I get iridium if I'm going to keep the car a long time, or I go with coppers if I'm not.
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