Common sense tells me that idling your car for 5-10 minutes will reduce your gas mileage per tankful just by running the car that long.
[ http://www.google.com/search?hl=en&q=car%20warm%20up%20myths&bav=on.1,or.&um=1&biw=1680&bih=925&ie=UTF-8&sa=N&tab=iw#sclient=psy&hl=en&biw=1680&bih=925&source=hp&q=car+warm+up+myths&aq=f&aqi=&aql=&oq=car+warm+up+myths&pbx=1&bav=on.1,or.&fp=9cba23060d8c177 ]
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