Re: Air recirculation
I have always used the air recirculation with the A/C. SGL is right about that the chilled air can be recycled back through the AC instead of taking in warm outside air.
And I have always used the air recirculation with the heater. Just the reverse - warm air can be recycled back through the Heater instead of taking in cold outside air. Yep, foggy windows can be a problem with the recirculation.
All the other times - I do not use the air recirculation. Air will come through the vents through the normal course of driving with the vent open even if the fan is not on.
As for running tests on mpg - I have not tried my normal route with the air recirculation on. My guess is that it would not make a statistical difference either way.
The five biggest factors in my view:
Temperature
Driving Style (which includes Speed)
Traffic
Terrain
Distance of Trip
Number six would be Tire Wear. My mpgs are better this year versus last and the other five factors have remained relatively constant. (Difference 35,000 additional miles on the tires = less rolling resistance.)
Lifetime Average 31.992 mpg 43,200+ miles
Since March 1, 2008 33.370 mpg 5,200+ miles
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