To optimize cooling and MPG, go windows down for a bit and then turn on the AC
Like lots of other people, we have been interested in establishing the fastest way to cool a car (lower the interior temperature to a comfortable level). When we set out to answer this question, we realized we needed to answer three others for members of the Driving2Save community: (1) what is the fuel economy penalty for using the AC to cool a car (compared to windows down and no AC); (2) what is the best approach to cooling your car and maximizing your MPG; and (3) is there a speed at which the drag from having the windows down creates more of a fuel economy penalty than using the AC with the windows up?
Based on our research, testing, and analysis, the short answers to our questions are: (1) the fuel efficiency penalty for using your AC is very high, especially when idling or driving at slower speeds; (2) the best way to cool a car (considering time, temperature/comfort, and fuel penalty) is to roll the windows down for a brief period of time, then turn your AC on a low setting and roll the windows up; and (3) at highway speeds, we recommend rolling windows up for safety and comfort and using AC as necessary to cool the car.
There are some pretty interesting pieces of advice floating around the internet for how to cool down a warm car interior. Our favorite involves rolling down one window and then walking to the other side of the car and violently opening and closing the door to push the hot air out of the car. Even if you cooled your car this way, you would be so hot and bothered from the effort that you would be worse for wear. Even with other recommendations, there is little or no science to support them.
We decided to approach the question scientifically and designed a test to answer the precise question: What is the fastest way to cool your car — AC on and windows up vs. windows down and AC off? And we then assessed the fuel savings associated with rolling your windows down to cool the car at lower speeds, at least for a brief period of time.
In our research, we also noticed that there has been a debate about whether using your AC on a warm day with the windows up at highway speeds is more or less fuel efficient compared to having your windows rolled down and the AC off at highway speeds (where drag reduces fuel economy). The Society of Automotive Engineers (SAE) designed a test that partially answers this question. They found that using the AC consumed more fuel compared to rolling your windows down, at least at speeds below 68 mph. (The data suggested that at some point just above 68 mph, the reverse could be true, which is one of the many reasons why we recommend windows up on the highway).
We decided to run a test on a Honda Fit using a predetermined loop around Santa Barbara, CA. We used the following procedures to obtain our data:
- Heat the interior of the car up to 105 degrees (F) either naturally with the sun or with a heater
- Use a scientific thermometer to record ambient temperature
- Run the course with the windows rolled down, obeying the speed limit
- Record temperature every minute
- Repeat course with the AC on and windows up
- Repeat the above procedures, but heat the interior of the car up to 120 degrees (F)
Figure1: Test 1 with 105 degrees (F) start. Blue line represents windows down while the orange represents AC. Roughly 4 minutes pass before the cooling differential effects are equal between AC and windows down.
Figure2: Test 2 with 120 degrees (F) start. Blue line represents windows down while the orange represents AC. Roughly 2 minutes pass before the cooling differential effects are equal between AC and windows down.
From our data, we can clearly see that there were differences in temperatures between a car cooled by AC versus windows down. This was expected as the AC should be more efficient at cooling, but what was surprising was the relative rate of cooling provided by each method.
Table 1: Cooling rate in percent per 1 minute interval
|Cooling Rate (Percent of Original Temperature)|
As the table above indicates, the largest amount of cooling took place in the first two minutes of operating the vehicle. The AC was the most effective, reducing interior temperatures by 17-25% within one minute of operation, while windows down reduced between 9-17%. However, as we approached four minutes, the percentage plateaus and temperature reductions fall to 1-4% and the difference in temperatures are less than 8 degrees (F) between AC and windows down.
With this data, we determined that opening your windows for several minutes (4 to 7) to cool off your cabin before turning on the AC will provide significant cooling while saving you some money from reduced fuel consumption, at least on days where the ambient air temperature is not substantially above your desired cabin temperatures. (On hotter days, you may only keep your windows down 1 to 2 minutes to reduce cabin temperature before running the AC).
But how much money can you save? We performed an analysis while using the following assumptions:
- 3 minutes of windows down driving time before turning on AC
- 2 miles driven based on 40 MPH
- 23.9 MPG (Fleet average for the US 2015)
- $3 per gallon gasoline
- 55-60% fuel economy penalty for running the AC at 100% while idling; 22-27% fuel economy penalty for running the AC at 100% at 40 mph
- Week calculations only account for 5 days to be consecutive
- Year extrapolation account for 6 months of AC use
We also used two parameters for this analysis – one where someone gets into his/her car 3 times a day and another where they get into it 10 times a day. Based on our calculations, 3/day person will save $0.41/week, $1.67/month and $10.04/year whereas, the 10/day person will save $1.39/week, $5.58/month and $33.47/year. Note, you can save even more if you use a sunshade because you will reduce your AC usage even further.
Conclusions regarding fastest and best ways to cool a car:
If you want to cool your car as fast as possible, there is no question that using your AC with vent open at higher fan speeds with the windows up, right from the start, is the fastest way to cool your car.
However, cooling your car this way imposes an enormous fuel penalty – running the AC on higher fan speeds dramatically increases fuel consumption when a car is idling and traveling at slower speeds. This costs you extra money at the pump. If you want to avoid this cost and achieve almost as much cooling in your car, roll down your windows and leave your AC off for a few minutes, then roll your windows up and turn the AC on a lower setting. To maximize your savings, use a sunshade every time you park in the sun.
Although these savings are modest, someone who takes frequent trips throughout the day and follows these tips could save close to a tank of gas each year. Additional benefits include reduced emissions and less time spent at the gas station each year. So remember the next time you get into your car on a hot day, roll your windows down for a few minutes and then turn on the AC.
When you get on the highway though, roll your windows up. It creates a safer and quieter cabin. If you are too warm, run your AC. At speeds above 68 mph, this may even save you money on gas by increasing your fuel efficiency (compared to putting your windows down and turning the AC off).
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Test 1: 105 degrees (F) Interior Temperature
|Windows Down||Full AC|
|Ext Temp (°F):||73||Ext Temp (°F):||75|
|Minute||Temperature (°F)||Minute||Temperature (°F)|
Test 2: 120 degrees (F) Interior Temperatures
|Windows Down||Full AC|
|Ext Temp (°F):||80||Ext Temp (°F):||82|
|Minute||Temperature (°F)||Minute||Temperature (°F)|