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Does Car A/C Use Gas? (Explained By A Mechanic)

Does Car A/C Use Gas? (Explained By A Mechanic)
Fact Checked and Reviewed by: Kris Jackson, ASE-Certified Mechanic
Kris Jackson has been a mechanic since 2010 after graduating from UTI. He’s worked with several master mechanics and holds several ASE Certifications. You can read more about Kris here.

Saving money on gas just makes sense. After all, it doesn’t matter who you are, you don’t want to waste gas or money. But there are dozens of ways to make that gallon go a little longer and some are much more effective than others. So is it worth a little sweat to leave the A/C off or turn it down to save cash? 

Does the A/C use gas and is it enough that you should care? 

Yes, your car’s A/C uses fuel because it’s powered by the engine. It can vary a lot by car and driving conditions, but you can expect this to reduce your fuel economy by 1 to 4 mpg. This comes out to around 10 to 46 cents per 30 miles. That comes out to about 20 to 92 cents per hour when driving at an average speed of 60 MPH.  

50 cents an hour isn’t going to be worth sweating in the heat for most folks, but you can see how this can add up quickly over long distances or many hours of driving. It can also add up even faster if it’s already hot outside and according to the EPA, your car’s A/C can drop fuel efficiency by 25% on very hot days

That’s the quick answer and should be enough to help you decide if it’s worth cranking the A/C or not. But we’re going to cover a lot more so you can make a better estimate for your specific vehicle and figure out when rolling down the windows might actually make more sense (and when it will end up costing you more). 

Let’s get started!

How Does Your Car’s A/C Work? 

There’s a complex process that converts refrigerant from a gaseous state into cool, comfortable air via your A/C conditioner compressor. This compressor is powered by the engine via a serpentine belt. When the compressor is turned on, the engine provides the power and since the engine runs on gas (or other fuel) that means a lower mileage. 

That’s a much more simplified version, but eventually, that cool air makes its way into the cabin to cool you off. 

This process is much different from your car’s mpg-friendly heating system or the lower wattage demands of your seat warmers. The A/C process requires more energy, more complex components, and therefore more fuel.  

If you want a deeper understanding of this entire process, this quick video does an excellent job explaining how your A/C removes heat and gives you cool air: 

How Much Does A/C Impact My Engine In Terms Of Horsepower?

Your engine doesn’t have infinite power, so running your A/C takes power from the engine and uses it to run the A/C compressor. 

On average, your car’s A/C compressor will use around 4 horsepower. Of course, this can vary by the size of the vehicle, outside temperature, the efficiency of the A/C, and more but that’s a good working assumption for most modern vehicles. 

Now let’s assume you have an engine with an output of 150 horsepower. In that scenario, running the A/C will use 2.67% of the engine’s total power. Thinking about it in these terms can really help us understand how A/C would impact gas mileage since it’s obvious that our engine uses gas. The A/C makes a much bigger impact on fuel than other vehicle accessories like the defroster

As a side note, this impact on overall horsepower can even impact how quickly your vehicle accelerates and this video does a great job explaining that with a real world testing showing 7% decrease in performance with the A/C on full blast: 

Okay, So How Much Can I Save By Not Using My Car’s A/C?

Sticking with our assumption that the typical car A/C unit uses 4 horsepower or around 3,000 watts of power we can convert this into a dollar amount to figure how much we can save by skipping the A/C.

Running the A/C, and the cost in horsepower or wattage, usually leads to a drop in gas mileage of around 1 to 4 mpg. That’s based not only on our math here, but Consumer Reports actually completed several real world tests which confirmed the typical drop of 1 to 4 mpg. 

So what does this mean for you? 

  • If there’s a reduction of 1 MPG, the low end of our range, the additional cost is $0.03 per mile.
  • If there’s a reduction of 4 MPG, the high end of our range, the additional cost is $0.12 per mile.
  • That means the average of these two is $0.075 (or 7 and a half cents) per mile. 

Let’s look at this for a longer journey across 100 miles: 

  • With a 1 MPG reduction, the additional cost because of the A/C would be $3.00.
  • With a 4 MPG reduction, the additional cost would be $12.00.
  • The average would be about $7.50. 

There are certainly plenty of variables here but with a mix of real world testing and solid math, this is pretty close to what you should expect. 

Should I Roll Down The Windows To Save Money? 

You don’t want to just sit, sweat, and suffer in your hot car to save a few bucks. 

So what about rolling down the windows instead of turning on the A/C?

This can work but only at lower speeds or if you’re driving in the city. At higher speeds, leaving the windows open creates so much drag that your engine has to work harder and ends up burning more fuel than if you just left the windows closed. This is especially apparent in larger vehicles that often have bigger windows and can therefore create more drag.

This is a major debate online and even the Myth Busters got involved to test it. A big reason why this is so contested is because there are a lot of variables to consider. 

But I’m going to try and wrap up the debate! If you’re driving faster than 60 MPH it’s best to roll up the windows and run the A/C. If you’re driving below 40 MPH and doing a lot of stop and start, there’s a good chance drag will be limited and rolling down the windows can save cash.

If you’re in between 40 to 60 MPH, the best option will depend on how much stopping and starting you’re doing. More stopping and starting will favor windows down while highway driving will favor the A/C. 

Does The Car’s Fan Use Fuel? 

What about using the fan without the A/C on- will that use fuel too? 

Yes, but it’s a very small amount of fuel. Your car’s blower fan runs off the electricity that it gets from the alternator. The alternator is powered by the engine and as you know the engine runs on gasoline so powering the blower fan requires fuel in a roundabout way. 

However, it’s a very small amount and much less than would be used by the A/C. 

We can assume that the blower motor on the highest setting will take around 20 amps of power (that estimate is on the higher end) which translates to 0.32 horsepower. Remember, your car’s A/C took 4 horsepower so we already know it’s going to be much cheaper to run just the blower motor for the fan. 

If your engine produces 100 horsepower (to make things easier) and uses 8 gallons of fuel per hour doing so, it uses 0.08 gallons for each horsepower per hour. So, for our added 0.32 horsepower will take 0.0256 gallons of gas per hour. If we’re looking at a gas cost of $3.00 per gallon we end up with an hourly cost of $0.0768 to run the fan.

That means running the fan alone, without the A/C, will cost only a few cents per hour. However, the fan alone isn’t going to make a big difference on a hot day but it won’t make a big impact on your mileage either. 

Overall, using the A/C in a car, based on a 4-horsepower consumption, can cost between $1.80 to $7.20 per hour in fuel depending on the drop in mpg. In contrast, running the car’s blower motor without A/C, which consumes approximately 0.32 horsepower, has a significantly lower fuel cost, estimated at only a few cents per hour.

Avoid Turning Your A/C On And Off (In Most Cases)

Those of you who are especially clever and frugal might consider turning your A/C on, cooling off a bit and then turning it off. 

If the A/C is off for less time, you should be saving fuel right? 

Not exactly. 

Depending on the type of compressor you’re using, turning on your A/C requires a small surge of energy to fire up the compressor and start the process. This little surge is more energy-intensive than the continuous running of the compressor so turning your car’s A/C on and off actually ends up requiring more energy (and therefore gas) overall.

Your car’s A/C also has to work a little harder to “catch up” with all the on and off switches. 

What Else Can Cause Your A/C To Burn More Gas Than Usual? 

Besides frequently cycling your A/C on and off, there are a few other things that can lead to higher fuel consumption for your A/C but not all of them are under your control. However, don’t stress this too much, and using gas is part of driving your car. Everything from your car’s radio to your car’s headlights uses gas so don’t get too obsessed with your mpg!

Malfunctions and Breakdowns

Just like any machine, your car’s A/C system can break down. But unlike many other aspects of your vehicle, the A/C system doesn’t get as much routine care. There aren’t regular maintenance filters and while there’s something called a receiver dryer that collects moisture it isn’t changed regularly unless there’s a problem. 

Lack of refrigerant is a problem, not a maintenance thing. The A/C system is a sealed system and there shouldn’t be any leaks in it. Usually when there is a leak it just won’t work very well. 

Extreme Outside Temperatures

Using your A/C on maximum during extremely hot days can place a higher load on the engine. The hotter it is outside, the harder your A/C works to cool down the inside of the car. There’s not much you can do here but if you’re really worried about saving some cash and literally going the extra mile, consider driving in the morning or evenings when it’s cooler. 

Using the ‘Max’ Setting Continuously

While it’s tempting to blast the A/C on the coldest setting, doing so constantly can put a continuous load on the engine, leading to increased fuel consumption. It’s okay to max it out a bit, but keep it steady when you can and about 20% below the max setting. 

Closing Thoughts

Saving money by managing your car’s A/C isn’t as straightforward as you might think at first.

Yes, a car A/C uses gas but you don’t want to turn your car into a sauna just to save a few bucks, and rolling down the window can actually be more costly if you’re going fast enough! So before you start sweating to get more out of every gallon, make sure you consider all the variables! Ride safe! 

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