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Discussion Starter #1
Power curve on my 2004 6.0l puts max torque (560 ft/lbs) at 2000 rpm. Read in an RV forum that max efficiency is attained at max torque for a diesel. Looking for the sweet spot for hour after hour of interstate driving pulling the 5th wheel. My truck hits 2000 rpm at 70 mph.

Thanks, Friz
 

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"resident smarty pants"
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Guarantee you that max fuel economy isn't going to happen at 70 mph - you have wind resistance to consider.

Unloaded, the best fuel economy seems to be closer to 55 with my truck. Not sure about the point for best efficiency when towing heavy.
 

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Project Shamu
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In a round about way, kind of, but like mentioned above, you are fighting wind resistance, as well as many other drawbacks that are out of your control. While towing your aerodynamics are out the window, they are garbage, so in almost every way, going slower will result in higher MPG, even if you get outside of "peak torque".
 

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The volumetric efficiency of your engine will peak around where you are producing the highest amount of boost, but that may not be the spot of best fuel economy. Air friction increases with the square of air speed (or in this case the speed that your truck is traveling through the air including any head winds). However... the real bug is that the power required to move your truck through the air increases with the air speed cubed. As LT.Dan points out, you'll will probably find that the best fuel economy will be at a lower speed where you may not be operating at the engine's best volumetric efficiency.
 

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More than the friction energy loss is the opposing force you have to overcome from the direct application of the wind "in your face" (impact of air mass against the vehicle surface area).
 

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Discussion Starter #6
Thanks for the insight. Possibly too many factors in my equation. Max MPG should be removed to make the equation true. At any rate I did not consider the importance of air resistance/mass to trucking on down the road. Now if I could find a portable stasis to wrap my rig in Max MPG could be re-included. Thanks, shipmates, I learned a lot.
 

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Something else to consider is engine loading. I can't remember his name off the top of my head (he's got posts in the mileage section), but there's a guy who has done everything he can to improve mileage, including lowering his truck and adding a secondary overdrive gearbox. He's definitely done his homework. One of the things he experienced is that when the engine is just on the verge of lugging it is getting max MPG. He uses a stick, so he can do that easy enough. With an auto, not so much. The only other advice I'd throw in is side skirting. If you know you're not going to be going over big bumps etc, see if you can design some side skirting for the 5th wheel, and perhaps a bigger air dam for the truck. Check out the mileage section for some good reads.
 
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you bought a diesel truck so you could tug around and enjoy your 5th wheel...life's too short to worry about fuel economy....TROMP ON IT, AND LET 'ER RIP!!!
 
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