Behind the Forecast: Why your car thermometer is often wrong

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

Behind the Forecast: Why your car thermometer is often wrong
Meteorologist Tawana Andrew explains why the temperature displayed on your car dashboard may be wrong in this week's Science Behind the Forecast. (Source: Pixabay)

LOUISVILLE, KY (WAVE) - The temperatures you see on your car’s dashboard are a LIE! OK, now that I’m done being melodramatic, let’s break down why they actually are wrong sometimes and why idling your car to warm it up is actually bad for your vehicle.

Usually, temperatures are measured with a mercury thermometer. Mercury expands and shrinks to certain levels as heat is added and removed, respectively. Our vehicles don’t have typical thermometers; they have thermistors. While thermistors are much like thermometers, they actually measure changes in electrical currents that are a result of heat being added or taken away. The issues with thermistor readings don’t come from the instruments themselves but actually where they are located. In most cases, thermistors are placed behind the grill at the front of the vehicle.

The air temperature that we typically think of is measured in a controlled environment to limit interference from nearby surfaces radiating heat and other outside sources. Thermistors are quite sensitive to heat being re-radiated from the ground or roadway; the temperature readings are incredibly localized and therefore sometimes inaccurate.

Now, thermistors aren’t useless. They are much more reliable on cloudy days because the sun’s heat is not being re-radiated off of a roadway; this is also why they are more accurate at night and when traveling at higher speeds.

Thermistors are not sensitive enough to tell small differences in temperature, particularly when temperatures are close to freezing. This is quite important during the winter months because just a few degrees separate icy, slick roads from just wet roads.


While most of us like to be warm while driving in our vehicles during the winter (I myself often wish I had heated seats), idling your car to warm it up is actually quite bad for it. A 2007 study showed that, on average, Americans believed it was OK to let a car idle for almost four minutes as it warmed up during sub-freezing winter temperatures. Data from this study suggested that 10.6 billion gallons of gasoline were wasted each year as cars idled.

Here’s the kicker. Cars are already less fuel efficient in colder temperatures, according to the Environmental Protection Agency and the Energy Department. Estimates show that most vehicles are 12 percent less fuel efficient at 20°F than at 77°F.

Idling your vehicle, while it makes it more comfortable, increases fuel consumption and the emission of pollutants. Think about it this way: You’re using gallons of fuel without getting a single mile in return. An EPA report showed that warming a vehicle up for just 10 minutes increased fuel consumption by 12-to-19 percent. Experts say that a car will warm up much faster when it’s being driven and that it should be warmed up no more than 30 seconds before we drive off.

Science Behind the Forecast: Why your car thermometer is often wrong

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