BBowers said:
Hey buddy, you might want to review some texts on pumping losses in internal combustion engines.
No offense, but the fact that diesels are more efficient due to no throttling losses, and higher compression, (and higher BTU per gallon, which is really just fuel specific, not engine specific) is widely known and advertised, even on
www.fueleconomy.gov .
My point is that there is a downside to running a 5. 9 liter engine empty at 2200 rpm and comparing its specific fuel consumption per horsepopwer to a 14 liter semi engine running at 1200-1400 rpm at 75% of its rated capacity, with pretty high exhaust temperatures, and having to shift gears to keep it in that efficient 200 rpm band.
Air is made up of 80% non combustible gasses, and they only cool the fire needed to push the pistons. More air dilution=poorer fuel economy. Too little air=meltdown of engine metals + high NOx emissions.
Your solution to best fuel efficiency is to run the engine at a specific target EGT, such as 800F (depends on particular engine), and shift whatever gears it takes, and whatever rpm and boost it takes to stay there. (usually that means very low rpm in town and cruising) We lack the gears and this is why in the not too distant future most cars/trucks will have CVT (continually variable transmissions) that will rev up the engine just enough to supply the power needed at the moment at the most efficient rpm/boost required. (Also, variable displacement, and variable valve timing/duration is becoming more common. )
That is if the EPA was not involved. Since there is no mandated mpg for heavy pick-up trucks, they are tuned to pass EPA and still produce advertised HP, not fuel economy. In 2007 the NOx limits are so unrealistically low that the only way to
maintain fuel econmy and meet EPA is to use catalytic urea injection into the exhaust. If you want a really efficient diesel wait until those come out. I think the MB bluetec engines will have that, and european engines do have it. Of course, there is the extra expense of that chemical bottle that will need to be refilled. But it allows the timing to be advanced to the optimum efficiency point, then clean up NOx in the exhaust.
Also, a semi in town, (and at 70-75 mph) only gets 4 MPG, and accelerates at a fraction of what most of us do.
Attention: TDR Forum Junkies 
I agree that if the laws of proportionality apply then my truck should be getting at least 25 and more like 35. My 05 only has 6500 miles but it has had several 17 MPG tanks but not even close any more. everything is hand calculated so I feel your pain.
