What are you comparing the cars reading with ? Is it the temp broadcasted on the radio, or maybe the temp on the bank sign ? I don't think that their very accurate and the temp on the radio is in whatever part of the city the station is.

The only way to find out the right temp would be to park the car in the shade and have an accurate thermometer to compare it with. Even this would be difficult because to get a good reading on the car it should be moving, otherwise your engine heat would have an affect on the sensor. I always feel that if I'm within + - 3 degrees of a bank sign temp or radio temp that's about the best I'll get.