Unfortunately that is a question only you can work out,using a tank to tank calculation ..That will tell you what you are actually doing.As to what it should be doing ,is a difficult one ,the shape of the the body ie brick affects it,lack of a roof spoiler would be a minus ,size of engine ,age of vehicle and milage on the engine ,how it is driven ..The list is endless ,sorry to be so negative .
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lorries/trucks.