Behind the Forecast: Winter’s chilling effect on cars

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Updated: Nov. 22, 2019 at 8:38 AM EST
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LOUISVILLE, Ky. (WAVE) - The cold winter air does not get along with our vehicles and wreaks havoc on everything from our tires to the engine. But why?

Well, we have to bring the Ideal Gas Law (also known as the Equation of State) into the conversation. It is PV = nRT. This equation demonstrates the relationship between pressure (P), volume (V), and temperature (T). The term n describes how many gas molecules are in a volume while R is a constant called the ideal gas constant.

For our tires, the equation demonstrates that lower temperatures (T) lead to a decrease in pressure (P). Tire pressure will drop around 1 pound per square inch (psi) for every 10°F drop in temperature. A fully inflated tire at 80°F is 6 psi under-inflated at 20°F.

Under-inflated tires can potentially fail in icy or snowy conditions. Find your vehicle's optimal tire pressure on the label on the inside of the driver's side doorframe.

The cold air also messes with car batteries for a few reasons: it thickens the oil inside the battery and reduces the amount of energy it can produce.

A cold engine must work harder to push around the cold, syrup-like oil, which makes it harder for the engine to start. Synthetic oils are recommended for colder climates for this reason; they stay liquid at lower temperatures.

The chemical reactions inside of a car battery slow down at cold temperatures. Since the battery then produces less energy, the starter motor does not have enough to work with to start the engine, forcing it to crank slowly.

A conventional gasoline car's gas mileage is 12% lower at 20°F than at 77°F, according to experts.

While cold engine oil and issues with battery performance are some of the most obvious causes for lower fuel economy in the winter, here's a list from the U.S. Department of Energy of a few other reasons why our fuel economy drops in colder months.

  • It takes longer for your engine to reach its most fuel-efficient temperature. This affects shorter trips more since your car spends more of your trip at less-than-optimal temperatures.
  • Heated seats, window defrosters, and heater fans use additional power.
  • Warming up your vehicle before you start your trip lowers your fuel economy; idling gets 0 miles per gallon.
  • Colder air is denser, increasing the aerodynamic drag on your vehicle, especially at highway speeds.
  • Tire pressure decreases in colder temperatures, increasing rolling resistance.
  • Winter grades of gasoline can have slightly less energy per gallon than summer blends.
  • Icy or snow-covered roads decrease your tires' grip on the road, wasting energy.
  • Safe driving speeds on slick roads can be much lower than normal, further reducing fuel economy, especially at speeds below 30 to 40 mph.
  • Using a four-wheel drive uses more fuel.

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