Dealer Brochure on 2008 HHR SS
#166
#170
The "450" number was just for promotion. The mileage is the same. I have seen mine state over 500 numerous times (strickly highway of course). The "480" number comes from 30mpg w/16 gallons. They know now the HHR can get the advertised mileage. They could only speculate that two years ago.