It is a 26 gal tank. Same tank used on the gas engines. On the diesels when it apears that it is empty you really have about 4 gallons left. The major reason for this if you run out of fuel you will have to prime the fuel lines in order to get all the air out of the lines. You do this with the priming pump mounted at the top of the fuel filter and it can become a real pain to do this. I believe GM doesn't want all those phone calls to dispatch and pay for their Road-Side Assistance program. So, they really stress not to run the truck out of fuel in the manuals. I personally wouldn't test my theory, but I believe it to be true.