OK, you have a more or less, a fixed load that you want to transport a given distance in a given amount of time. You have two options, a 6.9L engine and a 7.3 L engine. Both can do the job, but you may have to work the smaller engine harder to meet the time requirements. It would stand to reason, the larger engine would require more fuel to just maintain idle due to the larger parts that are rotating. It stands to reason that the larger engine should be able to do the same amount of work without having to work as hard. So, the slightly increased fuel consumption should not be a concern, because if it is working easier, you will be more relaxed when driving. This should be the big plus of the larger engine. I believe that Rabbit Man has said it all....
