Lesson 7: Air Conditioning and Ventilation
In Lesson 1, you were implored to keep your windows closed and your air conditoner off. This is and will always remain the absolute best advice insofar as fuel economy is concerned but it doesn't really help you out if you're frying in your car. Now, there's a lot of misinformation out there concerning the economy of opening windows versus using air conditioning in differing vehicles and at differing speeds. The fact is that you can never know which form of ventilation suits you best under what driving conditions in your particular vehicle without specifically testing all the relevant scenarios.
In order to get you started, I've gone ahead and used my ScanGaugeII to test a variety of typical scenarios in my car, an automatic 2.3L Mazda3 hatch. I conducted my tests by traveling in the same direction along 2km long stretches of flat road or freeway depending on the speed under test and using cruise control to ensure constant speed. Fuel economy over the course of each individual test was provided by my ScanGaugeII's trip computer. To whatever extent possible, road and weather conditions remained constant. In the case of open windows tests, windows were completely open. In order to set up a cross wind, I completely opened the driver's side window and the right, rear passenger's window. Air conditioning tests were performed at the lowest fan setting and coldest temperature setting with air recirculated. Idle tests were performed in gear (as opposed to in park) and results are in liters consumed per hour. At-speed results are reported in L/100km.
| Idle (LPH) | 50km/h (L/100km) | 90km/h (L/100km) | |
|---|---|---|---|
| No Windows or A/C | 0.93 | 5.2 | 6.4 |
| Driver's Window Only | N/A | 5.1 | 6.3 |
| Cross Wind | N/A | 5.2 | 6.6 |
| A/C | 1.57 | 5.8 | 6.4 |
Two things are evident from the table above.
First, that using your air conditioning while idling can result in a monstrous impact on your fuel consumption. Yes, that's almost a 70% increase in fuel consumption! Just imagine all the times you've been caught idling in gridlock or at a red light with your air conditioner on. Now think about how much extra it was costing you!
Second, that opening your windows is not the "drag" on fuel economy that some pundits would have you believe. The theory suggests that opening windows damages your vehicle's aerodynamic profile and therefore your fuel economy to such an extent that you're better off with air conditioning, but that simply may not be true. Even at high speeds and with a cross wind set up for maximum ventilation, the impact on my vehicle's aerodynamic profile did not appear to substantially impact fuel economy.
If my results are to be trusted, then it would hardly seem to matter how you ventilate your car at freeway speeds because the impact just isn't statistically significant. You may prefer to go with air conditioning, though, since opening your windows at high speed creates a hellacious and uncomfortable amount of wind and noise.
At city speeds, there is a statistically significant impact observed when using air conditioning. The difference, at over 11%, is substantial enough that it can seriously damage the results you hope to achieve by ecomiling so it is recommended that you keep your air conditioner turned off.
If you absolutely must use your air conditioning under circumstances where it will significantly impact your fuel economy, be active in regulating it. Turn off your climate control system and manually operate your air conditioner by employing the following impact-minimizing air conditioning tips:
- Always operate your air conditioner with recirculated air. This will cool your car down more quickly and cheaply.
- Always direct your air conditioning at your person. This will make you feel cooler and therefore demand less use of the air conditioner.
- Turn off your air conditioner when idling and turn it back on again when you're up to speed.
- Try turning off your air conditioner periodically while leaving the fan on. You will continue to get cold, recirculated air out of your vents for quite a while and you will feel cool and refreshed without needing your air conditioner on constantly.
- Never turn on your air conditioner with your windows open.
Once again, the best advice insofar as saving money is concerned is to never use your air conditioner and to keep your windows closed. No matter what weather or vehicle-specific variables are involved, this is always going to be the most economical way to drive. If you need to ventilate your car, though, opening your windows may be preferable to air conditioning. You may wish to test the above scenarios in your car in order to determine how to most economically ventilate your car when you absolute need to but, once again, the best way to save money and truly embody the ecomiling technique is to keep your air conditioner off and your windows closed.
Move on to Lesson 8 for some advanced fuel-saving techniques.
Lesson Quicklinks
Lesson 1: Get Started by Doing the ObviousDisclaimer: It is of critical importance that you always place safety first and fuel economy a very distant second. Although ecomiling may be an engaging, exciting and money-saving challenge, never place fuel economy above your safety or that of other road users. Obey all local traffic laws.