For starters, they're not actual thermometers.
The thermometer in your car isn't actually a thermometer. Technically, it's a thermistor, a device that measures temperature based on the change in electrical current that temperature can cause, instead of the expanding or contracting of a liquid like mercury. As the Washington Post points out, in most modern cars, the thermistor is placed in the front of the car, somewhere behind the grill. This is a big problem with the accuracy of that temperature you see on your dashboard. Coronation Thermometer
Anyone who has walked on sand on a hot day or felt the heat coming off cement knows that the sun heats the ground hotter than the air. The thermistor in your car is near to the ground, meaning it can pick up the heat that the ground radiates. This means that often, on hot days, the temperature you're seeing in your car is higher than the actual temperature of the air, because your car is picking up a mixed message, the temperature as its influenced by hot asphalt.
At night, the accuracy of the thermistor is better, because it's not picking up as much heat from the ground. This is also true on a cloudy day when the sun has not heated the ground as much, or when the car is traveling very fast.
Unfortunately, your car's thermistor is also not very helpful in the winter. Because it isn't accurate to always distinguish between above and below freezing, it can't help you figure out if the road may be coated with black ice. With all these limitations, car thermometers are a fun toy, but not very useful if you actually are looking to find out the temperature. You're better off checking the weather on your phone. As long as you're not currently driving, anyway.
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