On average how much does a brand new car depreciate as soon as it is driven off the dealership lot?

Many have suggested that once a car has been driven off the dealership's lot, it loses 15-20% of its value. During the second year, it loses another 15-20% of the remaining value. Keep in mind that the car does not lose 15-20% of the total value during the second year, but rather off the remaining value.