8. Air-Conditioning
Consumer Reports found that running the air conditioner reduced mileage by 1 mpg in its tests involving the Mercury Moutaineer and Toyota Camry. There was no measurable loss of mileage with the windows open.
So keeping the windows down and the A/C off as much as possible will help conserve fuel and reduce CO2. Because A/C use influences fuel consumption, the EPA is altering its 2008 model year test procedures to include this loss of efficiency in its mileage reports.