Ariel Williams, works at Earth
Updated Mar 3, 2016
Originally Answered: Is there a speed where using the AC in your car is more efficient than having your windows open?
The Myth Busters tackled this one.
At 45 mph it was more efficient to drive with the windows down. The car's fuel efficiency computer cannot be relied upon because it bases itself on airflow not consumption. They instead did a real ride till your dry test and the car with the windows down won.
Windows down vs. air conditioning
"Urban puzzle": it is more efficient, on a hot day, to run with the A/C on and windows up than to run with windows down (b/c of increasing car's drag).
Computer-based mpg measurements: * 11.7/11.8 with A/C on and windows up * 11.7/11.8 with A/C off and windows up * 11.3 with A/C off and windows down
So, according to the computer, it's better to use A/C with windows up.
This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you're-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).
Jamie's A/C car ran out of gas first -- Adam's windows down SUV ran for another 30 laps -- completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down.
Arjun Singhal, Automotive groups, safer driving
Answered May 23
I think it used to be a reality thirty years ago, that cars became less efficient with the air conditioner on. This is for many reasons.
Overall engine capacity and efficiency: Let’s pick an average family car from the early eighties, like a Premier Padmini - would have only about 1100cc, 47 bhp power. And it weighed approximately 900kg. The gearing ratios were such, that it allowed for a slow pick up, pushed by a rear-wheel drive mechanism with transmission losses, lower systemic efficiency and inadequate cooling mechanisms in the engine, making it consume a lot of fuel. An average family saloon today, like a Toyota Corolla would have 1.8L engine churning out tons of torque, 130+ bhp weight approximately the same. Not to mention the aerodynamics of the vehicle, which means getting to 60 kph in a Premier Padmini would eat up nearly 40–50% of the engine capacity, while in a Corolla, this might be just about 25–30%. Now if you add an AC to either of these capacities, that takes away power from the system, the net efficiency of a Premier Padmini will drop drastically if the engine is to operate at 60%-70% of power, while for a Corolla, you will still be only at 35% engine capacity requirements which will be efficient for the car of its engineering design. This is considering the AC takes away 5 bhp of power from the system.
Airconditioner efficiency: Engineering progress in air conditioning design have made air conditioners more efficient. Better insulation in cars, circulation systems, materials that absorb less heat, reflective coatings on the glasshouse that reduces heat inside, fabrics and upholstery that heat up less, make the system more efficient. Thus, lesser load from airconditioning in a modern car, than compared to an older car.
Aerodynamics: Older cars were less aerodynamic than newer cars. Which means, most of the energy from the engine was spent in countering the drag created by motion of the vehicle.
In a newer car which has drag coefficient as low as 0.2, you are using only 20% of engine power countering drag. But aerodynamics are calculated only with the windows up. Not with windows down. However, in an older car, the drag coefficient could be as high as 0.5, which would be quite the same as the drag with the windows down or up. Which means, in a newer car, you will end up spending much more engine power if the windows are pulled down, just to counter additional drag. If the drag coefficient goes up from 0.2 to 0.3, you’re already spending 10% more power countering that. Hence, savings from keeping the airconditioning switched off will be naturally countered by the inefficiency due to open windows.
Additional systems on newer cars: Newer cars have compressors running on the engine for providing power assistance to steering, braking, and other automated components which are all driving from the serpentine belt in the engine. Since this is already taking away some power from the engine, taking away another 2.5%-3.5% power will not drain the system entirely.
So net net, a modern car will probably not have much of a fuel economy difference with the airconditioning off, or on. Yes, you might encounter higher fuel efficiency in winters, but that could also be the effect that you are using lesser energy in keeping the engine cool, and the air being thicker in winters allows for better combustion of fuel with higher efficiency as compared to summer air. The airconditioner may have little to do with the fuel consumption in summer vs. winter months.
Tony Wang, MS Computer Engineering, Boston University (1988)
Answered Thu
As you noted, there are many variables/scenarios to consider, so this is a fairly difficult question to answer. However, there have been at least some experiments, which may give us an indication of which is more efficient.
The Myth Busters did a well-known test by measuring air flow to the engine as an estimate of mileage. In that experiment, they detected no significant difference driving an SUV. They weren’t satisfied with the results and later retested, this time starting with 2 identical SUV, an empty tank, adding 5 gallons and then driving around a track until they ran out of gas. This time they drove at 45 MPH, and the car with no AC and windows down ran about 15% further, a significant amount.
Of course the faster you go, the greater the wind drag, and some have said that above 50–55 mph, AC would win out. However, GM and SAE did a test in 2004 and found that windows down used less gas than AC at 31, 50, and 68 MPH for both a sedan and SUV. http://www.sae.org/events/aars/p...
Results will certainly vary with different cars and driving conditions, and modern cars are probably different, too. Too many factors to give any definitive answer, but it doesn’t appear to be a clear win for AC as many people think.
Jack Ciri, I'm a car enthusiast. I test many new cars.
Answered May 23
It depends on the speed you’re going. If you are going slow, having your windows down would be more efficient than having the air conditioning on and windows up. If you’re on the highway it’s possible that having your windows up and AC on would be more efficient.
There are other factors here that contribute too. How big is your engine? How aerodynamic is your car (would the windows down greatly affect the aerodynamics?) The most efficient way to drive is with windows up and AC off, but that might not always be convenient/comfortable.
On a car with a small engine (my old Focus for instance) it made a measurable difference if the AC was on or off. My highway fuel economy dropped by 3–4MPG just from having the AC on. On my Challenger, there is no measurable difference with it on or off because the car is so inefficient in any circumstance. My Focus was also very aerodynamic so opening the windows would hurt it as well. My Challenger is like a rolling brick so aerodynamics are never really a consideration, windows up or down… (though most likely the Challenger IS more aerodynamic with windows up)
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