Temperature is a measure of average kinetic energy. Kinetic energy is defined as a body being in motion. We measure air temperature using a thermometer, which usually is made out of mercury. When it is heated, the mercury expands, when it is cooled, the mercury contracts.
Cars use a thermistor instead of a thermometer. This means that it is measuring the temperature by a change in electrical current instead of by mercury, like a thermometer would. Due to this it can give an inaccurate reading.
The car thermistor is usually located near the bottom of your car. This means it is close to the roadway. Black top can absorb heat, so in the summer, your car will read a higher temperature compared to the actually air temperature. In the winter, if snow or ice is under your car and evaporating, it can lead to a lower temperature compared to the actual air temperature.
Official temperature readings are usually taken at airports. The thermometer at the airport is placed in a wooden shelter with white paint. It is typically 4 feet above the ground with ventilation to let air move through. It is also positioned so no direct sunlight can hit the thermometer when the door is open. This is why the official reading can be off compared to your thermometer at home, or what your car is reading, because official air temperature is taken in the shade.