I know there are a lot of people wondering about where the "sweet" spot is on the cummins for mileage and I am here to tell you that there is no sweet spot. Speed kills. I have ran numerous tests and proven that every 5mph gain costs you 2mpg. A while back I was testing this theory and on a 70mph trip I got 21mpg, on a 60mph trip I got 25mpg, at 55mph I was at 26.6mpg. So I theorized that doing 65 on this 700 mile trip would yield 23, and I was exactly right. I got exactly 23mpg. Less RPM=less injections per minute. Less speed=less wind to push out of the way. It's simple, and it works. I know some people out their are claiming 26mpg at 70mph or something crazy like that but if thats the case, why aren't they posting 30+mpg numbers when they do 60! :confused:

Very good! :thumbsup:

There is something to be said about fuel quality also...........however, I agree with you 126.43%!!! The best I've gotten in my 04.5 on extended trip was 21.2mpg running 65mph. Coming back from North Dakota in Nov. '08, with 3 guys, 2 dogs and pheasant hunting gear in my truck; I got 20.2mpg running 75-80mph. Both of these trips there was very light winds.The NoDak trip got me thinking though. My truck was "loaded" pretty good coming back. Probably 9000lbs+ GVW. However, I think that made the truck sit lower and actually "cut" the air better. That's my theory anyhow..........go ahead and cut it up!!!!

As the speed the truck moves through the air doubles........the wind resistance is "SQUARED". ie: there is an exponentially HUGE increase in wind resistance drag with increased speed. There are other factors as well but I think this is probably the biggest. This is something we were taught in flight school back during my days with Uncle Sam wearing a green pickle suit. :biggrin:

I got a problem. The tank before this one was done at 70-75mph favoring the 75, and somehow I got 24MPG. Not sure how I pulled it off. But I got mad at that result so I said if it gets 24 at 75 it will get something even dumber at 80mph. Needless to say, I am at 1/4 tank and only went 550 miles :eek: But I also skipped the 2 stroke on the 80mph run. So now I don't know what the hell to think. All my other results have been dead nuts to my 2mpg/5mph rule. Obviously, need to do more testing!

I've done a similar run like that too... But I've never been able to repeat it twice. For me I blame it on terrain (mostly downhill) and winds (with tail wind)... some of these weird factors that are unseen and have a huge effect on the MPG numbers...

I did notice my EGT were staying around 600 the whole time. Might have been the 11f ambient keeping it there. Don't think I ever saw it over 700, even when I was doing 80. Think it is downhill from NY on down to columbus though.

Give you a clue... A trip from My place to Riggins, ID is 15 miles in distance. But the terrain is downhill. I lose 1,000 feet of elevation from my house to Riggins, ID so in the fact is it looks some what flat but your running down hill the whole time. Give you a idea... From New Meadows, ID proper to Whitebird, ID is all down hill... That is over 75 miles of road that is down hill. I've learn how to take advantange of this natural free fall... So if I started in New Meadows and went to Whitebird I could pull near 32-35 MPG on the SG II because of the downhill run clipping along at 60 MPH... But now reverse this and now driver skill kicks in. Now how do you climb the same hills now and keep the numbers from falling off as fast. Keeping them in the 20's...

Now how do you climb the same hills now and keep the numbers from falling off as fast. Keeping them in the 20's...

..drive my truck under 80mph We got a hill on the way to college I ran the GPS on and it is a 500ft rise in about 1/8 mile. Roommate rode with me up it one day as I cruised up it at 60mph with 10psi boost and he said "you must go absolutely insane when your in a gasser after driving this so long" He has a 4.6 f150 that, hell I don't think it can even hold 60 going up it even after downshifting, maybe if he floored it and it went into 2nd it would hold it.

45-50 mph seems to yield the best fuel economy for my truck. The fuel economy goes down from there with increased speed. Definitely a far right lane only speed.

There is something to be said about fuel quality also...........however, I agree with you 126.43%!!! The best I've gotten in my 04.5 on extended trip was 21.2mpg running 65mph. Coming back from North Dakota in Nov. '08, with 3 guys, 2 dogs and pheasant hunting gear in my truck; I got 20.2mpg running 75-80mph. Both of these trips there was very light winds. The NoDak trip got me thinking though. My truck was "loaded" pretty good coming back. Probably 9000lbs+ GVW. However, I think that made the truck sit lower and actually "cut" the air better. That's my theory anyhow..........go ahead and cut it up!!!!

Where at in ND did you go Rooster huntin?

Where at in ND did you go Rooster huntin?

Stayed in Medina........which is about 30 miles West of Jamestown. Been going to your state every year since '99 duck and goose hunting. This was the first time strictly for pheasants.