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A/C vs. Open Windows
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Proper use of the air recirculation button can help you save fuel.
Chicago can achieve some grueling summer temperatures. While warmer weather is usually kind to the conscientious hypermiler, a Midwest heat wave can dismantle our best intentions.
One of the great debates among fuel efficiency afficionados is the use of air conditioning versus riding with the windows open. All agree that using your A/C while tooling around the city is a needless waste of fuel. Stop-and-go city driving is already a big drain on your fuel economy, and the A/C only makes your engine work harder, and your fuel consumption greater.
The debate starts at highway speed. Is there more drag on your engine at 65 m.p.h. with the windows closed and the A/C running? Or with the A/C off and the windows open?
Quite a few people and organizations have tackled this question, and for as many of the different groups and individuals, there are as many different conclusions.
Wayne Gerdes, the Father of Hypermiling, states that, economically speaking, the A/C should never be used, and cooling the passenger compartment of a car should be done by driving with the windows open, though he does advise discretion (and A/C use) where comfort and health issues are concerned. However, carjunky.com, October 29, 2006, claims the A/C should be used at highway speeds. And there are countless opinions of varying degrees in between.
Everyone's studies conclude the same things:
- with the A/C on below highway speeds — and especially in short-run, stop-and-go city driving — fuel efficiency is seriously diminished
- running the A/C at highway speeds cuts fuel efficiency by a minimum of 10%, and by more the faster you go
- running at highway speeds with the windows open cuts fuel efficiency by a minimum of 10%, and by more the faster you go
- the most economical practice at highway speeds is to drive with the windows closed and the A/C off. Also agreed is that, at the peak of summer, this practice would be impossibly uncomfortable.
The arguments center around determining which method wastes less fuel.
A few basic rules of thumb do apply where economical A/C use is concerned:
- upon first starting a car on a hot day, open all windows and vents and run the fan only (A/C off) at its highest speed to blow all the hottest air out (when possible, park your car in a garage or in shade so the interior isn't so hot when you get in)
- run your A/C with the "Recirculation" on. This closes the vents and cools only the air inside your car, and is not bringing in yet more hot air from outside the vehicle to cool.
- at highway speed, run your A/C at the highest fan setting until the passenger compartment is at a comfortable temperature. Then shut off your A/C (but leave the fan running) for a few minutes until you begin to feel uncomfortable. Following this cycle keeps you comfortable, but runs your A/C half the time or less.
As you move forward in your journey toward effective hypermiling, you will have to be your own judge in this matter. Experiment in your own car. Every different model of car, truck, and SUV has a different drag coefficient, and will respond differently than any others at different speeds.
As always, safety and your comfort should be your guides. Be realistic as to how badly you "need" the A/C. If you're on your way to an important job interview or client meeting, your appearance is very important. You want to stay fresh and free of perspiration. If you have health issues aggravated by heat and/or humidity, your comfort is paramount.
A/C vs. open windows resources:
-http://sustainablechoices.stanford.edu/actions/on_the_road/airconditioning.html
-http://www.bankrate.com/finance/auto/will-rolling-down-windows-save-fuel-or-not.aspx
-http://www.greenhybrid.com/discuss/f22/ac-vs-windows-open-mileage-test-3160/
-http://kwc.org/mythbusters/2004/11/mythbusters_boom_lift_catapult.html
More info about hypermiling, and Wayne Gerdes:
-http://www.cleanmpg.com/forums/showthread.php?t=1510
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