The computer in the HHR will compensate it's "Tune" for whatever octane you put into the tank. GM recommends premium in the 2.4 because that's what the engine is designed to use in order to get the maximum combination of peformance, economy, etc. By using lower octane you may experience somewhat less performance (only a dyno will tell) as well as fuel mileage. You also increase the risk of detonation. It's your engine so run what you like. I'll stick to GM's recommended fuel for my particular application.

 

ALL OF THE ABOVE

 

I don't know why I said function, just judging by my vehicular experience I go for performance and looks more.

 

Looks and functionality combined with a low price. My previous commuting vehicle was a 2002 K2500 Suburban with the Vortec 8100. I used to have a very short commute to work, but we moved to a rural area and now the commute is much longer. I was worried about wearing out the Suburban (which is the tow vehicle for our travel trailer) with high mileage more than getting better gas mileage. Better to wear out a $15K car then a $50K truck. I was initally going to buy a newer used car but the low price of the HHR (and GM card rebate of $3K) made it a good alternative to a (reliable newer) used car. Also since I do a lot of home woodworking, etc the HHR is useful for hauling 2x4s, etc.

We now have:

2006 HHR 1LT
2002 Suburban LT K2500 (with a few mods)
2002 Chevrolet Venture LT
1987 Mustang GT (with a few mods)

Steve