Turtle n Peeps wrote:
No, not at all Shiner.
It's that, PLUS many other things like total weight of the truck, surface area of the front end and even things like height of the truck. It's very difficult to compare one truck to another. Here is a good example. Ford changed the front air dam on the front of their F250 and picked up between 1/4 and 1/2 MPG JUST with changing the chin spoiler. That's a BUNCH! Just with a chin spoiler change on the front of the same truck.
HP is just a measurement of lifting weight in a set amount of time. That's why city mileage suffers so much. You're lifting that weight over and over and over for every stop sign or light you hit.
In any event, I started this thread to try to explain why adding power to the SAME exact diesel engine will not hurt your fuel mileage unless you use that power. This is the exact reason why tuners on diesels don't cost mileage. They add power by adding fuel and timing. If you don't use that power, there is no mileage penalty.
I am not disagreeing with you that adding more peak power to the same exact diesel engine will not hurt its fuel economy. I am however saying that a smaller displacement diesel will get better fuel economy than a larger displacement diesel due to many factors with one being less internal parasitic loss to not having to move larger engine internals.
One prime example of the is seeing the 4BT and 6BT run on the Dynos when I was at Cummins. The 3.9L 4BT is essentially a 5.9L 6BT with two cylinders cut off. It consistently got better fuel economy than the 6BT.
In another scenerio, a Cummins ISX 15L would not get the same fuel economy as a ISB 6.7L or ISF 2.8L does making 100 hp.
