8 cyl 6.0L SMPI Flex Fuel. 4 Wheel Drive! Extended Cab! How sweet of a deal is this! Just in, this good-looking 2013 GMC Sierra 2500HD comes with an 8 cyl 6.0L SMPI Flex Fuel engine and Automatic. It appears that the previous owner has always stored this vehicle inside and given it the extra attention that a magnificent vehicle like this deserves.