The high temperature today at Tucson International Airport was 106°, yet my car told me it was 113° on my way to work. Car thermometers usually deceive us, and so do bank signs on the side of the road. The instruments themselves are fine, and are usually accurate. It's the influence from the environment around your car that's the problem.
Car thermometers are actually called thermistors (thermal resistors). Basically, they are accurate temperature sensors that gives you a digital display of the temperature. Every car uses these; so a $70k car won't have a "more accurate" temperature reading than my '99 Volvo.
It has everything to do with the location of the thermistor...
Under the front bumper - This is the most common place for cars. It's out of direct-sunlight, and far enough away from the heat of the engine.
Under the rear bumper - Same concept as the front bumper, but probably better since it's farther from the engine.
Side mirrors - Away from the engine, readily available to the outside weather-elements.
The temperature reading can be influenced by the heat of the engine compartment, but it is usually higher because of the heat radiating from the pavement, or a nearby building. This is why official temperature readings from the National Weather Service come from stations that are at least 5 feet above the ground (either grass or dirt) and far away from any trees, buildings or structures that could influence the air temperature.
So, when you start your car you'll probably see the temperature is very high. After you drive around the temperature may drop to a more accurate reading. If a car is parked over snow, it may actually be lower when started.
What about bank-sign temperatures? Those thermometers are usually on a roof or near concrete which, as we know, is a lot hotter than the air itself.
Car temperature-readings will never be "official" because of the changing environment around it, but it is still a good ball-park estimate.
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