It varies, for the 2001 Chevrolet Silverado 3500 with 8.1L engine, fuel economy is 13 mpg city / 17 mpg hwy. Fuel economy is the distance a vehicle can travel on one gallon of fuel.
Calculate your car's MPG at fueleconomy.gov, the official US Government source for fuel economy information. Make sure to register online, so you will be able to save your MPG information and view, edit, or update it later.
Just recently, General Motors announced that the 2015 Chevrolet and GMC full-size SUVs will all improve upon the fuel economy of the outgoing models by nearly 10 percent. Find out more here.