Vehicle thermometers: Why they can be inaccurate

Aug 31, 2023

It has become the summer equivalent of the once ubiquitous food pics on social media feeds during this sweltering summer of record temperatures that has made triple-digit heat index readings routine.

And although all those pictures of digital dashboard car thermometers aren’t making the summer heat wave any easier to endure, there’s at least some reason for hope: The temperatures posted on your dashboard aren’t always that accurate, according to weather.com.

Things might be cooler than your car is telling you.

Because of the design of those vehicle systems, there are factors that can make the dashboard readings a bit higher than the temperature outside.

Here’s what to keep in mind as you’re wiping the sweat from your brow:

If you're looking for an accurate reading of the temperature outside, pay no attention to your car's thermometer, according to weather.com

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On a warm summer day, it displays a temperature that can be significantly higher than the actual temperature, perhaps as many as 10 degrees above the outside air temperature, according to meteorologists.

Temperature readings on digital car thermometers are “usually higher” than the actual air temperature, said Robert Haley, a meteorologist with the National Weather Service in Melbourne.

It has to do with how the car systems compare with the scientific approach to calculating temperature used by the National Weather Service, Haley said.

“How we measure the official temperature, we have our thermometers in an open space, often in a field,” he said. “We also place them in a housing, so they are protected from direct sunlight with ventilation, to be able to measure the ambient temperature in the sun.”

By comparison, a vehicle thermometer is placed somewhere within the chassis, where it is subject to factors that can influence the reading that range from potential heat from the engine to reflected heat from asphalt parking lots.

Temperature is commonly measured with a mercury thermometer. The liquid mercury inside the thermometer expands and rises when heat is added, or contracts and falls when heat is removed.

Although it's commonly called a thermometer, dashboard temperature readings in vehicles are calculated by a thermistor, which measures the change in electrical current because of heat added or removed. Although it’s an accurate device, an array of variables can influence the reading.

If an automaker places the thermistor on the front of the car behind the grill, it exposes the instrument's readings to re-radiated heat from the road surface. If you're parked in a shadeless parking lot, the thermistor will be heated up — a lot — by the surface beneath the vehicle.

Likewise, if a car is parked in the grass or under shade, that also can affect the reading.

It’s the same reason that the temperatures posted on bank thermometers stationed in black-top parking lots tend to be higher than the actual temperatures.

The thermistor works better at night because it's not picking up as much heat from the ground. This also is true on cloudy days when the sun hasn’t heated the ground as much or when the car is moving fast enough to allow air to circulate in the area around the device.