I don't have any proof or even real evidence that they do anything. I am going on what I have read on sites like Daox mentioned. Mitsubishi somewhere wrote that the mirage gets better gas mileage with the steel wheels & hubcaps than with the aluminum mag wheels because of a higher aerodynamic drag on the aluminum wheel.
I am trying to get the best gas mileage that I can, without doing expensive modifications. These hubcaps cost me $35.
If they work, it may take a year or so to get that money back. After that, it's like getting free money every time I drive.
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View my fuel log 2014 Mirage DE 1.2 manual: 53.1 mpg (US) ... 22.6 km/L ... 4.4 L/100 km ... 63.8 mpg (Imp)