3.1 LS 20-29 3.8 Z34 19-28 With a light foot, better mileage can be achieved.
Regular gas is just fine for a 1999 model year vehicle.
There is no such thing as a 2009 Monte Carlo
the sticker on mine says 19 - 28, it depends on how it's driven though
It is on top of the gas tank. You will have to remove the tank to service it.
The fuel capacity of a 2000 Chevrolet Monte Carlo is 17 gallons.
See link.
My 72 Monte gets 15 mpg.
in the gas tank
bad gas get octane booster
17.1
~ 24 MPG
It's located in the gas tank.