If you're looking for an accurate reading of the temperature outside, pay no attention to your car's thermometer.
On a warm summer day, it usually displays a temperature significantly higher than the actual temperature, and there are several reasons why. For starters, your car does not actually have a built-in thermometer, but a thermistor.
Most commonly, the temperature is measured with a mercury thermometer. The liquid mercury inside the thermometer expands and rises to a certain value when heat is added, and contracts and falls to a lower value when heat is removed.
A thermistor, on the other hand, measures the change in electrical current as a result of heat added or removed. The problem is not with your car's thermistor itself; in fact, thermistors are typically accurate, not to mention small and cheap to make.
The real problem is where the thermistor is located on your car. Most automakers place the thermistor on the front of the car behind the grille. This location exposes the instrument's readings to re-radiated heat from the road surface.
If you've ever walked barefoot on the beach or on a blacktop on a sunny day, you likely felt the re-radiated heat directly as your feet burned on the hot surface.
Car thermistors provide a better representation of nighttime temperatures, when the heating from the sun is lost. They are also more accurate on a cloudy day for the same reason, as well as when traveling at higher speeds and not sitting in standstill traffic.