I have about 2200 miles on my HHR (2.4 auto) and it looks like Shell has given me the best mileage so far. The first few tanks, I used Chevron 91 (~26.5 miles/gal, mixed), then Sams 91 (~24.5, mixed), then Shell 87 (~26.0, mixed), Shell 91 (30.0, more highway than normal). But I'm still gathering data. So far, Shell is looking sweet! BTW, I'm in the Ogden, Utah area.