It's a combination of the weight (or more specifically the rotational inertia), rolling resistance and width. The aerodynamics of the wheel face will also have a small impact. The EcoNetic uses narrow 14 inch steel wheels, with 175/65 low rolling resistance tyres and smooth wheel covers. This is a far cry from the 17x7 inch 5 spoke alloys and stickier 205/40 rubber on my Zetec.
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