Does Running the AC Use More Gas? What MythBusters Found

Yes—running a car’s air conditioning uses more fuel because the compressor adds load to the engine. However, when MythBusters tested AC-on versus windows-down, they didn’t find a one-size-fits-all winner: at higher speeds the aerodynamic drag from open windows can cost as much or more fuel than the AC, while at lower speeds AC tends to be the bigger penalty. In short, the show’s takeaway was that it depends on speed, vehicle design, and conditions, even though AC itself undeniably burns more fuel than not using it.

What the MythBusters Tested

The MythBusters team ran comparative fuel-economy trials with vehicles driven at steady speeds under two scenarios: windows up with AC on versus windows down with AC off. They repeated the experiment in more than one episode and with different conditions, aiming to measure how far each setup could go on a fixed amount of fuel and to observe how speed and aerodynamics change the outcome.

Initial Findings

In early tests, MythBusters reported that neither approach was universally superior. The windows-down car suffered from increased aerodynamic drag at higher speeds, narrowing or even reversing the apparent fuel-efficiency advantage over AC-on with windows up. At lower speeds, where drag matters less, the AC’s compressor load more clearly reduced mileage.

Revisit and Final Takeaway

When the show revisited the myth with different vehicles and speeds, results again varied. The team emphasized that vehicle aerodynamics, compressor efficiency, and speed are decisive factors. Their practical conclusion: AC does use more gas, but whether it uses more than driving with the windows down depends on how fast you’re going and what you’re driving.

Why AC Affects Fuel Use

Air conditioning is powered by a belt- or motor-driven compressor that compresses refrigerant, creating a continuous mechanical load on the engine (or on the traction battery in EVs and hybrids). In gasoline and diesel vehicles, that extra work translates to additional fuel burned. Modern systems cycle the compressor and can be more efficient in recirculate mode, but the load remains meaningful—especially in hot, humid conditions or during hard cooling.

Practical Guidance for Real-World Driving

The following tips synthesize what MythBusters demonstrated alongside broader engineering consensus, helping you balance comfort with fuel economy in everyday driving.

• At city speeds (roughly under 40–45 mph), crack the windows and keep AC use light; drag penalties are modest here.
• At highway speeds (roughly 50–65+ mph), close the windows; using moderate AC often costs less fuel than the extra drag from open windows.
• Use recirculate to cool more efficiently; it reduces the heat and humidity load on the system.
• Set a reasonable temperature and fan speed; “max cool” and high blower settings drive up compressor duty cycles.
• Pre-cool while driving, not idling; idling with AC wastes fuel without covering distance.
• Keep the cabin shaded when parked (sunshades, tint where legal) to reduce initial cooling demand.
• For hybrids and EVs, AC affects range but far less than open-window drag at freeway speeds; use climate preconditioning when plugged in.

Applied together, these habits align with MythBusters’ speed-dependent outcomes and can reduce fuel or energy use without sacrificing comfort.

Key Variables That Sway the Result

Several factors determine whether AC-on or windows-down is more economical in a specific situation.

• Speed: Aerodynamic drag rises roughly with the square of speed; windows-down penalties grow sharply on highways.
• Vehicle shape: Boxy SUVs and trucks suffer more from open-window drag than sleek sedans.
• Compressor efficiency: Newer, variable-displacement compressors are more efficient than older on/off units.
• Temperature and humidity: Hotter, more humid air increases AC workload.
• Cabin heat load: Dark interiors, big glass areas, and direct sun boost cooling demand.
• Vent settings: Fresh-air mode often forces the system to cool hotter outside air; recirculate eases the load.

Because these variables change by trip and by vehicle, MythBusters’ mixed results make sense: the “winner” flips as conditions shift.

What Independent Testing Shows

Engineering studies and road tests outside the show generally find that AC use can reduce fuel economy by about 5–25%, depending on the vehicle and conditions, while open windows impose a drag penalty that becomes significant at freeway speeds. Many tests observe a crossover region—often somewhere in the 45–65 mph range—where closing windows and using moderate AC becomes the more efficient choice. The exact threshold varies by vehicle aerodynamics and climate.

Bottom Line

Running the AC does use more gas than not using it, but MythBusters demonstrated that whether it uses more than driving with the windows down depends largely on speed and vehicle aerodynamics. Keep windows down in slower, stop-and-go conditions; favor windows up and moderate AC on the highway.

Summary

MythBusters showed that AC unquestionably consumes fuel, but the “AC vs. windows down” efficiency question has no universal answer. At low speeds, open windows typically save fuel; at higher speeds, the drag penalty often makes AC the better choice. Your best strategy is speed-aware: windows down in the city, windows up with sensible AC on the highway.

What burns more gas, AC or Windows?

Using your air conditioner burns more gas than rolling down the windows at low speeds (below ~40-45 mph), but rolling down the windows uses more gas at high, highway speeds due to increased aerodynamic drag. The most fuel-efficient choice depends on your vehicle, speed, and outside temperature, with a good rule of thumb being to open windows in town and use the AC on the highway. 
At low speeds (city driving, under 40-45 mph) 

• Windows down are better: The aerodynamic drag from open windows is minimal at lower speeds, making it more fuel-efficient to open the windows than to use the air conditioning.

At high speeds (highway driving)

• AC is better: Opens in new tabAt highway speeds, the increased drag from open windows negatively impacts fuel efficiency more than the energy used by the AC system. 
• The AC system puts a load on the engine, Opens in new tabbut this load is less significant than the drag created by open windows at higher velocities. 

Key Factors to Consider

• Speed: Opens in new tabThis is the most crucial factor, with a general “magic number” around 40-45 mph separating when windows down or AC is more efficient. 
• Vehicle Type: Opens in new tabAerodynamic cars (like sedans) are more affected by drag from open windows, making the AC a better choice at lower highway speeds. 
• Outside Temperature: Opens in new tabOn very hot and humid days, the air conditioner is the only way to stay cool because open windows can make you hotter. 
• Driving Conditions: Opens in new tabFor stop-and-go city traffic, opening the windows may be more efficient, while for consistent highway driving, the AC is the better option. 

Does running AC really use more gas?

Does AC affect gas mileage? As a general answer, a Consumer Reports study found that, yes, running the AC does reduce gas mileage. Here are some tips to help you balance your fuel consumption with comfort. Generally, the air conditioner uses gas, so use it wisely.

What wastes the most gas in a car?

The biggest fuel wasters in a car are speeding, aggressive driving (rapid acceleration and braking), unnecessary idling, and carrying excess weight. Other factors include poor tire maintenance (underinflated tires), skipping engine maintenance, and excessive use of the air conditioner at highway speeds. Driving at very high speeds greatly increases air resistance, which is a major factor in fuel consumption.
 
Driving Habits

• Speeding: The faster you drive, the more air resistance your car faces, causing your engine to work harder and use more fuel. 
• Aggressive Driving: Frequent, hard acceleration and braking waste fuel because you lose the momentum you built up. 
• Idling: Leaving your car running when parked uses a significant amount of fuel per hour, especially with the air conditioner on. 
• Excessive Weight: A heavier vehicle requires more energy to move, so cleaning out unnecessary items from your car can help save gas. 
• Short Trips: Making very short trips can reduce fuel efficiency because the engine doesn’t reach its optimal operating temperature. 

Car Maintenance

• Tire Pressure: Opens in new tabUnderinflated tires increase rolling resistance, forcing the engine to work harder. 
• Engine Maintenance: Opens in new tabSkipping routine maintenance, such as replacing air filters or spark plugs, can lead to decreased engine efficiency and higher fuel consumption. 

Other Factors

• Air Conditioner and Drag: Opens in new tabUsing the air conditioner puts a power draw on the engine. At highway speeds, rolling down windows to get air can cause more drag and thus more fuel consumption than using the A/C. 
• Car Issues: Opens in new tabThings like dragging brakes can make the engine work harder and consume more fuel. 

Does air conditioning affect gas mileage in Mythbusters?

Yes, using your car’s air conditioner consumes more gas, though the impact varies significantly with speed. At lower city speeds, the AC’s engine load has a more noticeable effect on fuel consumption. However, at highway speeds, the increased aerodynamic drag from open windows becomes a greater concern, often making it more fuel-efficient to use the AC with windows up. 
How AC Usage Affects Fuel Economy

• Engine Load: Opens in new tabThe AC compressor puts an extra load on the engine, requiring more fuel to operate. 
• Aerodynamic Drag: Opens in new tabOpen windows create significant wind resistance (drag), which forces the engine to work harder and use more fuel to maintain speed. 

This video explains how AC affects fuel efficiency and the impact of aerodynamic drag: 1mBuild Your OwnYouTube · Oct 26, 2021
When to Use AC vs. Windows Down 

• City Driving (low speeds): Opens in new tabIt is generally more fuel-efficient to use the AC and keep your windows up. The drag from open windows at these speeds can outweigh the engine’s effort to run the AC.
• Highway Driving (high speeds): Opens in new tabIt becomes more fuel-efficient to use the AC with the windows closed. The increased drag from open windows at highway speeds is more detrimental to fuel economy than the load of the AC system.

MythBusters’ Findings

• The MythBusters tested this myth and found that the optimal solution depends on speed. 
• They discovered that for their specific test at 45 mph, running the AC with the windows up was more fuel-efficient than having the windows down. 

You can watch this video to see the MythBusters’ experiments and results: 59sMythBustersYouTube · Jul 5, 2025
Key Takeaway
To maximize fuel efficiency, use your AC with the windows up at highway speeds and consider rolling down the windows at slower city speeds, especially in stop-and-go traffic.