Does driving with the windows down consume more fuel?

Yes—at higher speeds, driving with the windows down typically consumes more fuel because the extra aerodynamic drag forces the engine to work harder. At lower speeds, the penalty is small, and open windows can be as efficient as, or more efficient than, running the air conditioner. The exact crossover depends on speed, vehicle shape, and how aggressively the A/C is used.

How open windows and A/C affect efficiency

Rolling down windows disturbs the vehicle’s airflow, increasing aerodynamic drag. Drag rises with the square of speed, and the power needed to overcome it rises roughly with the cube, which is why the penalty grows quickly on the highway.

Air conditioning, by contrast, adds a mechanical or electrical load. In modern combustion cars with efficient, variable compressors, A/C typically costs a few percent at steady cruise but can climb into the high single digits or more in extreme heat or stop‑and‑go traffic. In EVs, HVAC energy use can be modest on mild days but substantial in very hot or cold conditions—often more impactful at low speeds where aerodynamic loads are small.

Put simply: at city speeds, open windows are usually fine; at freeway speeds, the drag from open windows can outweigh the A/C load.

What determines whether windows or A/C uses more fuel

Several factors influence whether it’s more efficient to drive with windows down or use the A/C. Understanding these helps you choose the better option for the conditions.

• Speed: Below roughly 35–45 mph (55–70 km/h), the drag penalty from open windows is typically minor. Above about 50–55 mph (80–90 km/h), drag rises quickly and can exceed the fuel or energy use of moderate A/C.
• Vehicle shape and size: Boxy SUVs, trucks, and vehicles with roof racks or open beds suffer a larger drag increase from open windows than sleek sedans or hatchbacks.
• How many windows and how far they’re open: Cracking one window slightly or using rear windows can reduce turbulent drag versus driving with both front windows fully down.
• Climate control settings: “Max A/C,” low temperatures, high fan speeds, and fresh‑air (non‑recirculate) modes increase energy use versus moderate settings and recirculation.
• Traffic and temperature: In hot city traffic, frequent compressor cycling and heat soak can make A/C relatively costly. On the open highway, steady A/C loads are smaller while window‑drag penalties grow.
• EV considerations: At low speeds, HVAC can dominate energy consumption; at high speeds, aerodynamics dominate. Seat ventilation or targeted cooling can reduce HVAC demand in EVs and hybrids.

Because these variables interact, the most efficient choice changes with conditions. The “crossover” from windows to A/C usually lands in the mid‑speed range but varies by vehicle and weather.

Practical guidance for drivers

You can minimize fuel or energy use by matching your ventilation choice to your speed and conditions. The following steps provide a sensible rule of thumb for most modern vehicles.

1. Under ~40 mph (≤65 km/h): Prefer windows down or fan‑only; use A/C sparingly.
2. Between ~40–55 mph (65–90 km/h): Use a mixed strategy—slightly crack windows or use moderate A/C with recirculation; avoid fully open front windows.
3. Above ~55 mph (≥90 km/h): Keep windows up for best aerodynamics; use A/C at moderate settings, preferably with recirculate on.
4. Adjust which windows are open: If you need airflow, crack a rear window or use small openings to limit turbulence.
5. Optimize A/C: Use recirculation, reasonable temperature set‑points, and moderate fan speeds; pre‑cool while parked in shade when possible.
6. For EVs and hybrids: Prioritize seat coolers/ventilated seats and targeted vents; keep speeds steady to limit aero drag.

These techniques balance comfort and efficiency without requiring extreme sacrifices, and they work across a wide range of vehicles and climates.

What the testing shows

Independent road tests and engineering studies have consistently found that open windows measurably increase drag and fuel use at higher speeds, while A/C adds a variable but often smaller penalty on the highway. Results differ by vehicle, but many tests place the crossover—where A/C becomes comparably or more efficient than open windows—somewhere in the 45–55 mph (70–90 km/h) range. Modern variable‑displacement compressors and improved cabin sealing have reduced the A/C penalty in newer vehicles, reinforcing the general guidance above.

Bottom line

Driving with the windows down does consume more fuel at higher speeds due to added aerodynamic drag, often more than running moderate A/C. At lower speeds, the drag penalty is small, and open windows can be the more efficient choice. Match your approach to your speed and conditions for the best results.

Summary

Open windows increase aerodynamic drag, which matters most at highway speeds; A/C adds a mechanical or electrical load, which matters most in extreme temperatures and low‑speed driving. Below roughly 40 mph, windows down is typically efficient; above 55 mph, windows up with moderate A/C is usually better. In between, use a blended approach—small window openings or light A/C with recirculation—to balance comfort and efficiency.

Is it bad to run car AC with windows down?

However, if the air conditioner runs simultaneously, opening the windows could make it harder to cool the car down. Keeping the A/C on and the windows open isn’t particularly economical. Running the air conditioner puts additional strain on the engine, which consumes fuel.

Does driving with the windows down burn more gas?

Driving with the windows down burns more gas than using the air conditioner at highway speeds because of the increased aerodynamic drag, but it can save fuel at lower city speeds where the drag is minimal and the engine’s load for the AC would be more significant. A general rule of thumb is to keep windows up and use the AC on the highway, and to open the windows for city or stop-and-go driving. 
At Highway Speeds

• Open windows increase drag Opens in new tabThe faster a car moves, the more the windows disrupt the vehicle’s streamlined shape, creating significant air resistance (drag). 
• AC is more efficient Opens in new tabThe extra effort the engine must make to overcome this drag with open windows uses more fuel than the load of running the AC. 
• Sedans and SUVs are affected Opens in new tabA 2004 SAE study found that open windows reduced fuel efficiency by 20% on a sedan and 8% on an SUV at highway speeds. 

At Lower (City) Speeds

• Drag is less of an issue: The minimal drag from open windows doesn’t impact fuel efficiency as much at lower speeds. 
• AC engine load is higher: The engine has to work harder to power the AC compressor at lower speeds. 
• Windows are more efficient: In city or stop-and-go traffic, it’s more fuel-efficient to lower the windows to let in fresh air. 

Key Takeaway

• Use AC on the highway: keep windows up and AC on for best fuel economy on the highway. 
• Use windows down in the city: open windows at lower speeds and in city driving for better efficiency. 

Is it more efficient to have AC on or windows down?

While all this might seem like a minor issue, getting it wrong can have a notable impact on your fuel economy; using AC can increase your fuel consumption by as much as 10% but opening the windows at higher speeds can increase fuel consumption by as much as 20%.

Can I sleep in a car with windows closed AC on?

It can be fatal as carbon monoxide, which is extremely poisonous emits from the engine of a car in such a scenario. The gas may suck into the car’s cabin by the air conditioning vent and the unsuspecting occupant may die while sleeping inside the vehicle.