Fuel Economy in Hot Weather: AC, Heat, and Real-World MPG
- FuelEconomy.gov states using AC can reduce fuel economy by more than 25% under city conditions.
- At higher speeds, open windows can increase aerodynamic drag and reduce fuel economy.
- The best choice depends on speed, trip length, and how hard the vehicle has to work to cool the cabin.
What We Know (Sourced)
FuelEconomy.gov explains that using air conditioning can reduce fuel economy by more than 25% under city conditions. It also notes that at higher speeds, driving with windows down can increase aerodynamic drag and reduce fuel economy.
FuelEconomy.gov's hot-weather guidance also suggests practical steps like parking in the shade or using sunshades to reduce cabin heat buildup, which can reduce how hard the AC has to work.
Separately, EPA notes that fuel economy label values are produced using standardized testing for comparisons, but real-world results can vary due to conditions like speed, temperature, and accessory use.
Why Hot Weather Can Reduce Fuel Economy
For gas vehicles, the biggest hot-weather factor many drivers notice is AC load. The compressor draws power, and that power ultimately comes from fuel. For EVs, AC still uses energy — but it's electricity from the battery.
Hot weather also increases your incentive to idle (for example, to keep the cabin cool), and idling burns fuel without moving the vehicle. Related: idling fuel use.
AC vs Windows: The Tradeoff
FuelEconomy.gov frames a practical rule: at higher speeds, open windows can increase drag and reduce fuel economy, while at lower speeds the drag penalty is smaller and AC load can dominate.
That means the "right" choice depends on your scenario:
- City and stop-and-go: AC can have a larger MPG penalty, especially during short trips when the cabin is very hot.
- Highway speeds: Open windows can increase drag, so moderate AC may be more efficient than wide-open windows.
If you want a deeper AC-focused breakdown (including how to think about it above 50 mph), start here: does air conditioning use more gas? and speed and fuel economy.
How to Estimate the Cost Impact
The simplest way to quantify hot-weather impact is to compare two fuel-economy values:
- A baseline MPG for mild-weather driving.
- A hot-weather MPG for the same route while using AC.
Translate that MPG change to dollars using cost per mile:
- Cost per mile (fuel) = price per gallon ÷ MPG
- Monthly cost = miles per month × cost per mile
Related: cost per mile and commute cost budgeting.
What's Next
- Reduce the cabin heat load: park in shade, use sunshades, and crack windows when safe.
- Avoid unnecessary idling: if you're parked, consider turning the engine off when safe and legal.
- Measure your own delta: log MPG for 2–4 weeks during hot weather so you can budget accurately.
If your main concern is summer EV efficiency, track kWh/100 miles and your effective $/kWh. Related: kWh per 100 miles explained.
Frequently Asked Questions
Is AC always worse than driving with windows down?
No. FuelEconomy.gov notes that at higher speeds, open windows can increase aerodynamic drag and reduce fuel economy. The best option depends on speed, how hot the cabin is, and how hard the AC needs to work.
Why is the MPG penalty often bigger in city driving?
FuelEconomy.gov states AC can reduce fuel economy by more than 25% under city conditions. Stop-and-go driving and low speeds change the balance of engine load, cooling load, and drag effects.
Does hot weather reduce EV range too?
It can. AC uses energy from the battery. The magnitude depends on trip length, speed, and how hard the cooling system works. Tracking kWh/100 miles is the most direct way to see the effect.
Should I include AC use in my commute budget?
Yes, if hot weather is a regular part of your year. A practical approach is to budget using a summer MPG baseline and a mild-weather baseline, then compare.