Next time you get into a brutally hot vehicle that’s been sitting in the sun, be sure to thank the inventor of car air conditioning. It’s an amazing system that effectively creates comfortable cabin temps, but we all have that one friend or relative that refuses to use air conditioning since “AC lowers my gas mileage.” Does AC use gas? Read on to find out.
How does car air conditioning work?
First off, let’s have a look at how air conditioning works in order to understand why it could even be using gas in the first place. Start with the power source. Your home AC unit needs electricity from the power grid to operate. Car air conditioning cools and operates under the same principles as home AC, but the main difference is that car AC needs to draw power from an engine.
Then there’s the science. Refrigerant works on the principle of evaporation: When a liquid evaporates, it removes heat. It’s like sweating during yard work on a hot day. It’s not the sweat streaming down your brow that cools you, but the evaporation off your head that pulls the heat away. Vehicle refrigerant works in the same manner, with the system converting compressed refrigerant into a less dense gas, and pulling heat away during the exchange. Formerly R12 or Freon, modern AC systems use more environmentally friendly R134a refrigerant.
You’re probably somewhat familiar with the operation of a compressor. Like its name suggests, it compresses a gas. Air compressors pack air into a steel canister, which lets mechanics use it for air tools. A car AC compressor essentially performs the same task, just compressing refrigerant instead of air. The accessory belt on the engine spins a pulley on the compressor, which pumps heated refrigerant into the compressor. As the AC demand increases (gimme cold air now!), the drag on the pulley (and thus the engine) also increases. The compressor’s internal pistons compress the gas, often past 200 PSI. Under pressure, it passes off the hot gas.
Like a radiator, the condenser is a heat exchanger. Mounted in front of the radiator but behind the grille for maximum airflow, the condenser cools the compressed gas until it forms a liquid. This chilled liquid needs to get back into the cabin to cool you off.
But first, there’s the expansion tube. Also called an “expansion valve” or “fixed-orifice tube,” the valve adjusts the flow of refrigerant. Too much supercold liquid will freeze up the evaporator, while too little will mean your AC doesn’t blow cold at all. The expansion tube allows just the right amount of refrigerant based on the input from your AC settings.
Also called the “AC drier,” the receiver is for temporary storage, and is also a filter. The filtering removes any moisture and debris from the AC system, as water can damage the compressor.
Think of the evaporator core as a radiator in reverse. While the radiator on the front of your car carries a hot liquid to pass off engine heat to the outside air, your AC system’s evaporator pulls the heat out of the air and transfers it to the cold liquid refrigerant. The evaporator is where the magic happens in the AC system. A blower motor spins a fan that pushes the cold air into the cabin. The temperature exchange from hot air hitting the cold refrigerant boils the liquid off into low pressure gas, which takes the heat with it as it leaves the evaporator and heads back to the compressor. The result is that nice chilled air coming out of the AC vents.
Does AC use gas?
Reading above how the AC system works probably gave you some insight into the question: Does air conditioning use gas? This question has an easy answer. Yes, air conditioning, and anything else that draws power from the engine, uses gas. The less easy question is: Why?
Air conditioning uses gas because it is a system directly connected to the vehicle’s engine. Anything that draws a load from the engine—such as spinning the water pump, alternator or AC compressor pulleys—pulls engine power, in the form of additional gasoline, to operate the accessory. In this way, think of everything from your radio and headlights to your AC system as using gas. You can test this by checking the engine RPMs as you drive. Without AC, the engine might generate 2,000 RPM on a city street, but then kicks up to 2,200 RPM when you turn the AC on. Generally speaking, that increased engine speed will burn more gas.
There’s no easy way to say exactly how much gas the AC system uses without precise testing. Factors that will determine how much fuel the AC system uses are the age of the vehicle, condition and previous maintenance of the AC system, and the load setting used (the maximum setting obviously requires more energy than the minimum setting). Even among vehicles of the same class, manufacturers might use entirely different AC equipment. Just as not all four-cylinder engines achieve the same gas mileage, the difference between AC components contributes to the overall lack of an exact AC efficiency standard. Further confusing the answer, modern hybrids and EVs use an entirely different AC system, operating more like an electric heat pump for increased fuel efficiency.
How much gas does AC use?
While there are some variables such as ambient air temperature, parking in the shade or sun, or the level of tint on your windows, we do have a good general idea of how much AC use costs you in gas consumption. The US Department of Energy’s Oak Ridge National Laboratory tested just this hypothesis, to see if AC use caused fuel consumption to increase at idle, 40 MPH and 70 MPH. Scientifically controlling variables and measuring the results on their Ford Explorer and Toyota Corolla, Oak Ridge Lab delivered some cool results—pun definitely intended.
Comparing AC off with AC on at 100%, the lab showed the Ford Explorer used a whopping 55% more fuel at idle, a 27% increase above 40 MPH, and just a 14% increase at 70 MPH. The Corolla produced similar numbers, with a huge 60% increase in fuel use at idle, 22% increase above 40 MPH, and just a 9% increase at 70. The short version: it’s cheaper to roll down the windows at idle, but you won’t notice the increased gas use on the highway.
But does AC waste gas?
This is a difficult question, as “waste” is subjective to the driver and passengers. Is “wasting” 10% more fuel worth a comfortable cabin temperature? Most people would probably say yes. What about “wasting” 60% more fuel? Suddenly people start busting out their calculator app.
From the Oak Ridge study above, the Explorer continued to use more fuel with the AC on than with it off and the windows down, regardless of vehicle speed. On the other hand, the Corolla had a more efficient AC system, and only consumed more gas up to 75 MPH. Beyond that speed, aerodynamic drag with the windows down was more detrimental to gas mileage than spinning the AC compressor. Yes, AC uses more gas, but AC wasting gas depends on your vehicle, driving style and personal opinion. Keep in mind that windows down at 75 MPH also creates annoying buffeting and is seriously loud. AC on the highway seems like a no-brainer.
Tips for improving gas mileage (even with the AC on)
Sad about your gas mileage penalty, but enjoy your AC too much to give it up? No worries! There are several ways that may help maintain or improve your gas mileage, even while you’re making it snow inside your RAV4. Does car AC use gas? Who cares!
Check your tire air pressure
If that low tire pressure warning light is on and you keep driving, you’re burning money. Reduced tire pressure lowers fuel economy by 2% to 3%, with more impact at lower speeds. Low pressure also increases wear on the tires, and results in negative handling and stopping ability.
Use the recommended engine oil
If your owner’s manual recommends 5W-30, and you pour in heavier 10W-30, your fuel economy will suffer by 1% to 2%. That adds up over the next 5,000 miles.
Get an alignment when you get new tires/wheels
Or, any time you notice symptoms of misalignment. The camber, caster and toe all determine which direction the wheels are going, and being even a few degrees out of spec can negatively impact fuel economy by as much as 10%. Yes, this one also wears out the tires early if you don’t keep up with proper alignment specs.
Have a pro apply window tint
This thin layer of film provides heat and UV reflection, so the heat doesn’t enter the cabin in the first place. A vehicle with quality tint can reduce cabin heat by 43% and won’t need to use as much AC to stay cool.
Take a shadier route
Your AC has to work harder the more the sun heats up the engine compartment and cabin. The EPA says shaded surfaces are 20 to 45 degrees cooler than unshaded surfaces. If the time and distance are about the same, a tree-lined route offers some relief to your air conditioner.
Plan your trips for higher speeds
As you read above, AC at idle uses noticeably more fuel than at highway speeds. If your route has highway and city streets available, aim for higher speeds and reduced AC fuel consumption.
Start with a cool car
No, don’t go buy a Ferrari SF90 Stradale—just park in the shade. A shaded parking spot allows a cooler car’s interior to begin with, and the less your AC has to work to get it comfortable. Keep your car parked indoors when possible, and consider a car cover if not. When you’re on the go, park in the shade and consider a windshield sun reflector for when that’s not an option.
Drive with the windows down for the first minute
This quickly evacuates the hot air from the cabin, putting less demand on the AC system.
Set plug-in vehicles to pre-cool while charging
This lets you get into a comfortable cabin without reducing range on an electric vehicle or plug-in hybrid.
Activate “chill mode.”
That’s a real setting on Teslas, but you don’t need an onboard supercomputer to make you accelerate slower and drive in a generally more relaxed manner. Quick acceleration, speeding and aggressive driving can decrease gas mileage by 10% to 40%. Change your habits and you may save a good chunk of change.