My in car temp gauge is usually about 1 degree out from my weather station, but the car one only shows increments of .5. I can get in the car on a cool, but sunny day and the temp can be 20c, but as soon as you move off and the air gets to the sensor, the temp can go down to a realistic 12c. I presume the sensor wont be in the engine bay.....maybe under the boot area, away from the exhaust. You could try posting your question over on www.honestjohn.co.uk as this is a very active car forum (the forums are in the section called "the back rooms" and have a technical area which probably suits your question).