I did almost this exact swap on my old car, except I swapped to 15" Prius alloy wheels. Lightest weight factory option, plus a fairly flush clip on wheel cover with those. I used the black 10 spoke alloys which have the clip on 5 spoke silver wheel cover. I saw a MPG increase, but I also switched tires at the same time so your mileage may vary (literally). I had good luck with Michelin energy savers.
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