You can find this information:
* On the sticker inside the driver's side doorjamb: This sticker usually shows the fuel economy information.
* In your owner's manual: This should also list the EPA fuel economy estimates.
* Online: Search for "2006 Pontiac Grand Prix [engine size] MPG" (replacing "[engine size]" with the engine displacement, e.g., "3.8L"). Sites like fueleconomy.gov (US) or similar sites in other countries can provide this data.