I remember some GM advertising back in the day. They claimed that little spoiler created a low pressure area to help make the radiator more efficient by sucking air through it and under the car. (see Metro's linked articles...)
It'd be an easy thing to check...
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Best tank ever: 72.1 mpg in February 2005, Seattle to S.F.
New personnal best 'all-city' tank June '08 ... 61.9 mpg!
Thanks to 'pulse-n-glide' technique.
