The 2005 Grand Prix is suppose to run on 93 octane only (Preimum Only). If you are going to run (91) use a octane booster additive. I know the high cost of fuel is making us all wanting to use cheaper fuel. But that solved my wifes problem in her 2005 Grand Prix. She now gets around 23.5 mph in the city 28 - 30 Hwy