how to get the gas milage on my 2003 gmc 4x4 p/u with 5.3l exstended cab short box with 15 wheels

1) Fill up your gas tank completely with fuel. 2) Reset the "trip odometer" to 0 miles (usually a stick-looking button poking out of the gauges near the odometer). 3) Drive around as usual, or until bored. 4) Completely refill the gas tank with fuel. Note how many gallons it takes to completely refill the tank. 5) Note the mileage on your trip odometer, then divide that number by how many gallons of gas it took to refill the tank. The answer will be your fuel mileage, in miles-per-gallon.