Windows Down or Air Conditioning? What MythBusters Found—and How to Decide Today

It depends on speed and conditions: at highway speeds, using AC with the windows up is usually more efficient; at low speeds, windows down can be equal or better. The MythBusters tested this more than once and, along with engineering studies and agency guidance, the consensus is that aerodynamic drag from open windows matters more as speed rises, while AC’s energy draw matters more in slow, stop‑and‑go driving and extreme heat.

What MythBusters Actually Tested

MythBusters ran the “windows down vs. AC” trial in an early episode and again in a later revisit, attempting to measure which setup allowed identical vehicles to travel farther on the same fuel at steady speed. Their overall takeaway aligned with automotive engineering research: at higher speeds, the car with AC on and windows up tended to go farther, while at lower speeds the difference narrowed and, in some cases, windows down wasn’t worse and could be better.

How that squares with broader research

Independent testing and guidance from sources such as SAE technical studies, Consumer Reports, and the U.S. Department of Energy/FuelEconomy.gov echo the pattern. AC can cut fuel economy substantially in city traffic and during hot-soak cool‑downs, while open windows add aerodynamic drag that ramps up quickly with speed. In newer vehicles, more efficient AC systems and better aerodynamics tend to reinforce the “AC on for highways, windows down around town” rule of thumb.

Why the Answer Changes with Speed

At low speeds, aerodynamic drag is modest, so rolling down windows doesn’t penalize you much, but the compressor load from AC can. At highway speeds, drag increases with the square of speed and the power needed to overcome it climbs roughly with the cube, so open windows become a bigger penalty than a modern AC system running steadily.

Key variables that affect the result

The decision isn’t one-size-fits-all. The following factors can swing the balance between windows down and AC on in real-world driving.

• Speed: Below ~35 mph, windows down is rarely costly; above ~50 mph, open windows can exact a sizable efficiency penalty.
• Vehicle shape and size: Boxier SUVs and trucks suffer more drag from open windows than sleek sedans and modern crossovers.
• Outside temperature and humidity: The hotter and muggier it is, the harder AC must work, especially after a hot soak.
• AC system design: Newer systems, automatic climate control, and recirculation mode reduce compressor load versus older units.
• Traffic pattern: Stop‑and‑go amplifies AC’s impact; steady cruising moderates it.
• Sunload and cabin heat: Dark interiors, large glass areas, and direct sun raise cooling demand; shades/tint reduce it.
• Powertrain type: EVs and some hybrids use heat pumps and have different efficiency curves; windows-down drag still matters.
• Cabin pressure and partial openings: Cracking rear windows or using vents can help flow without the full drag penalty.

Put together, these variables explain why two drivers can see different outcomes on the same day—and why the MythBusters’ highway-focused results don’t automatically apply to short, slow urban trips.

Practical guidance you can use

Use these speed-based guidelines to balance comfort and efficiency in most modern vehicles, then adjust for your specific conditions.

1. Under ~35 mph: Favor windows down or fan-only ventilation; AC only if needed for comfort or defogging.
2. 35–50 mph: It’s situational. Light AC with recirculation or a mix of cracked windows can be efficient; avoid full open windows.
3. Over ~50 mph (highways): Use AC with windows up. Set a moderate cabin temperature and enable recirculation for best results.

These steps reflect the tradeoff that drag dominates at speed while compressor load dominates in slow, hot conditions. Comfort matters too—moderating settings usually preserves both comfort and efficiency.

Additional efficiency tips

If you want to minimize the energy hit from staying cool without sacrificing comfort, the following tactics are consistently effective.

• Pre-ventilate a hot car: Open doors/windows briefly to dump heat before you turn on AC.
• Use recirculation: Once the cabin is cool, recirc reduces hot outside air intake and compressor work.
• Aim for moderate temps: Setting 72–76°F/22–24°C cuts load versus “LO/Max.”
• Shade and reflect: Park in shade, use sunshades, and tint legally to reduce heat gain.
• Keep the system healthy: Clean cabin filters and well-maintained AC improve efficiency and cooling speed.
• For EVs/hybrids: Precondition while plugged in; use heat-pump “eco” modes and seat/steering-wheel cooling where available.

These habits help regardless of whether you choose windows or AC, and they’re especially impactful on short trips and in extreme heat.

How big is the difference?

Real-world impacts vary, but ballpark figures help set expectations. Government and independent testing suggest that running AC can reduce fuel economy by a few percent in steady highway driving and by much more—sometimes 10–25%—during short, hot, stop‑and‑go trips. At highway speeds with modern vehicles, driving with windows fully down can also trim efficiency, often on the order of several percent and potentially more for boxy vehicles. Your exact result depends on the factors above.

Bottom line

MythBusters’ results and current guidance align: use AC with windows up on highways; around town, open windows or light AC is fine. Modern cars and EVs make AC more efficient than it used to be, but physics hasn’t changed—drag at speed is costly. Choose based on speed, conditions, and comfort, and use recirculation and moderate settings to minimize the energy hit.

Summary

At high speeds, AC with windows up typically saves fuel compared with windows down; at low speeds, windows down can be as good or better. MythBusters’ tests support this speed-dependent outcome, which is reinforced by engineering studies and agency advice. For best results, tailor your choice to speed and conditions, use recirculation and moderate temperatures, and employ simple heat-management tactics like shading and brief pre-ventilation.

What trick gets the best gas mileage?

What are easy hacks to make my car have better gas mileage? The easiest one is to slow down. Driving slower will almost always save you gas, especially from about 40mph and up. Accelerate slowly, but steadily. No jackrabbit starts. Don’t stop or slow down if you can help it. slowing down to speed up again wastes fuel.

Does running the AC use more gas 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.

Is it better to have windows down or AC on?

Driving with the windows down is more fuel-efficient at low speeds (under 50 mph), while using the air conditioning (AC) is more fuel-efficient at high speeds (above 50 mph). At low speeds, the drag from open windows is minimal, and the AC’s power usage is a greater fuel drain. At highway speeds, the aerodynamic drag created by open windows significantly impacts fuel economy, outweighing the fuel consumed by the AC system. 
When to Roll Down the Windows

• Low Speeds: When driving at city speeds or in stop-and-go traffic (below approximately 50 mph), rolling down the windows is often more fuel-efficient than using the AC. 
• Better Air Quality: At low speeds, you can get fresh air without excessive noise or air pollution, and the airflow can help clear the cabin of chemicals from interior materials. 

This video explains how opening your windows at low speeds is more fuel efficient: 59sBjørn NylandYouTube · May 14, 2018
When to Use the Air Conditioning

• Highway Speeds: Opens in new tabAt higher speeds (above approximately 50 mph), the drag from open windows creates more resistance than the AC uses, making it more fuel-efficient to keep the windows up and the AC on. 
• Clean Air and Comfort: Opens in new tabAC provides a more pleasant and less stressful driving experience by maintaining a consistent, comfortable temperature and filtering out dust, fumes, and other pollutants from the outside air. 
• Defogging Windows: Opens in new tabThe AC system also removes moisture from the air, which is essential for defogging windows on humid or rainy days, improving visibility. 

What is the 3 minute rule for air conditioners?

The 3-minute rule for air conditioners is a guideline that suggests waiting at least three minutes after turning the unit off before restarting it. This pause is crucial because it allows the refrigerant pressure in the system to stabilize, which prevents damage to the expensive compressor and helps the unit operate efficiently. Ignoring this rule can lead to short cycling, increased wear and tear, higher energy bills, and a shortened lifespan for your AC system. 
Why the 3-Minute Rule is Important

• Compressor Protection: Opens in new tabThe compressor builds pressure to circulate refrigerant. Restarting it immediately after a shutdown places extreme stress on it, which can lead to costly damage or failure. 
• Pressure Equalization: Opens in new tabWhen the AC is turned off, the pressure within the system needs time to equalize. The 3-minute wait allows this essential pressure stabilization to occur. 
• Prevents Short Cycling: Opens in new tabQuickly turning the AC on and off without allowing the system to fully stabilize is known as short cycling. The 3-minute rule prevents this, ensuring the unit completes its cooling cycles efficiently. 
• Enhances Efficiency: Opens in new tabBy allowing the system to stabilize, you ensure it restarts properly, leading to better energy efficiency and consistent performance. 

How to Apply the Rule

• Wait Before Restarting: If you’ve turned your air conditioner off, wait for a full three minutes before turning it back on. 
• Use Smart Thermostats: Many modern smart thermostats have a built-in delay feature that enforces this minimum runtime, making it easier to follow the rule. 

In summary, the 3-minute rule is a simple yet effective practice to protect your air conditioner’s compressor, prevent short cycling, and prolong the life of your system.