Its designed to be MOST accurate when traveling at freeway speeds or after normal in town driving ,but not heavy stop & go . Think about it - if you had a thermostat and it was open to direct sunlight= incorrect reading , or if its hidden in the coolest part of the engine compartment --= Inaccurate reading , therefore you must place it where it get the most accurate "average" temp when the car is moving mine seems to be pretty damn good and always within 1-2 degrees of the weather reports . ??? I have Mazda 3 GT-HB 2.5 USA model .