Lesson: The Science of Winter Fuel Economy

This lesson is a small science class about your fuel tank. We’ll look at each physical reason cold weather costs miles, then at the habits that help. Even in mild Southern California, chilly mornings and short trips bring some of these effects into play.

The numbers first

Fuel economy tests show a conventional gasoline car’s city mileage is roughly 15% lower at 20°F than at 77°F, and as much as 24% lower on short 3- to 4-mile trips. ^{[FuelEconomy.gov]} Results vary by model, but FuelEconomy.gov says to expect a 10% to 20% loss in city driving and 15% to 33% on short trips for conventional cars. Hybrids typically lose 20% to 40% in city driving and 25% to 45% on short trips. ^{[FuelEconomy.gov]}

Reason 1: Thick, cold fluids

Engine and transmission friction increases in the cold because engine oil and other drive-line fluids are cold. ^{[FuelEconomy.gov]} Cold oil moves less freely, so the engine works harder until it warms.

Reason 2: A slower warm-up

It takes longer for the engine to reach its most fuel-efficient temperature. That hurts short trips most, because more of the trip happens before the engine is fully warm. ^{[FuelEconomy.gov]} This explains the bigger short-trip losses in the numbers above.

Reason 3: Heaters and defrosters

Heated seats, window defrosters and heater fans use extra power. Warming the car up by idling also lowers fuel economy, since idling gets 0 miles per gallon. ^{[FuelEconomy.gov]}

Reason 4: Denser air

Colder air is denser, which increases aerodynamic drag, especially at highway speeds. ^{[FuelEconomy.gov]}

Reason 5: Softer tires

Tire pressure decreases in colder temperatures, increasing rolling resistance. ^{[FuelEconomy.gov]} NHTSA gives the same reminder: as the temperature drops, so does tire pressure, so refill to the vehicle maker’s recommended pressure on the door-frame label. ^[NHTSA]

Reason 6: Winter gasoline

Winter grades of gasoline can contain slightly less energy per gallon than summer blends. ^{[FuelEconomy.gov]}

Reason 7: A tired battery

Battery performance decreases in the cold, making it harder for the alternator to keep the battery charged, and it reduces regenerative braking performance in hybrids, plug-in hybrids and EVs. ^{[FuelEconomy.gov]} NHTSA adds that gas and diesel engines need more battery power to start in the cold. ^[NHTSA]

Compare: what heat does

Hot weather can actually help fuel economy: the engine warms faster, summer gasoline can have slightly more energy, and warm air causes less drag. The catch is the A/C, which is the main reason mileage drops in hot weather and can cut a conventional car’s fuel economy by more than 25% in very hot conditions, especially on short trips. ^{[FuelEconomy.gov]}

Compare: what cold does to EVs

Electric vehicles feel the cold most. FuelEconomy.gov reports EV fuel economy can drop roughly 39% in mixed city and highway driving, with range dropping about 41%, and about two-thirds of that extra energy goes to heating the cabin. ^{[FuelEconomy.gov]} Without the cabin heater, EV fuel economy is only about 8% lower at 20°F than at 75°F, and range about 12% lower. ^{[FuelEconomy.gov]} That is why preheating while plugged in, and using seat warmers instead of the cabin heater, can extend range. ^{[FuelEconomy.gov]}

When it’s not just the weather

Seasonal drops are normal, but FuelEconomy.gov notes a check engine light can signal anything from a loose gas cap to a problem that lowers fuel economy, so have it inspected if it comes on. ^{[FuelEconomy.gov]}

Habits that help

• Park somewhere warmer, like a garage, so the engine and cabin start warmer. ^{[FuelEconomy.gov]}
• Combine trips so you drive less often with a cold engine. ^{[FuelEconomy.gov]}
• Skip long warm-ups; most manufacturers recommend driving off gently after about 30 seconds. ^{[FuelEconomy.gov]}
• Use seat warmers and defrosters only as needed, check tire pressure regularly, and use the manufacturer’s recommended oil for cold weather. ^{[FuelEconomy.gov]}
• Remove roof racks when not in use; a large roof box can reduce highway fuel economy by 6% to 17%. ^{[FuelEconomy.gov]}
• Drive smoothly. Aggressive driving can lower mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. ^{[FuelEconomy.gov]}

Bonus: severe weather makes it worse

On icy or snowy roads, tires lose grip and waste energy, and safe speeds can be much lower than normal, which further reduces fuel economy, especially below 30 to 40 mph. Using four-wheel drive also uses more fuel. ^{[FuelEconomy.gov]}

Calm next steps

1. Note your usual mpg on a mild week so you have a baseline.
2. Combine short errands into one trip on cold days.
3. Skip long warm-up idling; drive off gently after about 30 seconds.
4. Recheck tire pressure when the weather turns cool.
5. If mileage stays low once the weather warms up, have the car checked.

Frequently asked questions

How much does cold weather lower gas mileage?

FuelEconomy.gov says a conventional car’s city mileage is roughly 15% lower at 20°F than at 77°F, and up to 24% lower on short 3- to 4-mile trips. ^{[FuelEconomy.gov]}

Should I warm up my car before driving?

Most manufacturers recommend driving off gently after about 30 seconds; the engine warms faster while driving, and idling gets 0 mpg. ^{[FuelEconomy.gov]}

Does winter gas have less energy?

Winter grades of gasoline can have slightly less energy per gallon than summer blends. ^{[FuelEconomy.gov]}

Does hot weather lower mileage too?

Heat itself can improve fuel economy, but A/C use is the main reason mileage drops in hot weather. ^{[FuelEconomy.gov]}

Related reading

• Lesson: How Air Bags Work
• Lesson: How Hybrid Cars Save Fuel
• Lesson: How TPMS Sensors Work
• Lesson: Reading Tire Grades (UTQG)
• Ask a question

Sources

1. https://www.fueleconomy.gov/feg/coldweather.shtml
2. https://www.fueleconomy.gov/feg/hotweather.shtml
3. https://www.nhtsa.gov/winter-driving-tips
4. https://www.fueleconomy.gov/feg/driveHabits.jsp
5. https://www.fueleconomy.gov/feg/maintain.jsp

General education, not a diagnosis. See our disclaimer.