My 06 2.4 has been running 87 octane most of the time and gets ~30 mpg average. I have tried running 93 octane a few times (2000 miles) and saw no major mpg improvements and not worth the $0.20-0.25 per gallon price higher price. Although the car seemed to have more pep. MPG is really dependent on your right foot on the gas pedal. I'm no longer into stop light to stop light drag racing, and use a lot of cruise control on the highway. I'll dump some Marvel Mystery oil in the tank periodically.

I have even tried some acetane added to the gas for supposed great mpg improvement and saw no improvements.

I have tried pure gas (93 octane), but at $0.60/gal higher price, really not worth the higher cost.

The 06 HHR has over 111000 miles on the clock.

Not sure thats so much a result of lower octane, as much as lack of filtration of the fuel, both at the station, and of course the HHR doesn't have much in the way of fuel filtration.

Filtration at gas stations consist of strainer assembly to stop the big stuff and a 10 micron filter before it enters your tank. Newer stations do not have the dirt and water problems older stations do so if you are concerned try and find a newer station. Always look for clean looking stations because there owners are most likely to keep there fuel system in better shape.