I would not use anything less than 89 Octane
Try 87 Octane if it does not "ping" excessively you are OK If it does "ping" go to 89 Octane
GXPs come with the LS4 motor so with the compression ratio it's not recommended to go below 91 octane.
no. but it is recommened because of the higher octane its better for the supercharger and you will consume more power.
no
According to GM and www.feuleconomy.gov, the 2008 Pontiac Grand Prix has a 17 gallon gas tank.
My 2004 Grand Prix GT2 runs on 87 octane just fine.
I have a 2004 Pontiac Grand Prix GTP Comp G Supercharged, and it reccommends premium gas, but it does fine without it.
In the gas tank
It has an 17 gallon gas tank.
15 gallons
15
Sixty liters