Well, there are two gigantic differences in the physics of each engine that make one more efficient than the other (not taking into consideration transmission [auto vs manual]), but I guess we are just gonna call them the same

 

double post

 

As much as I like the Cummins 12 valve engine ( I own three of them currently) they just don’t get any better fuel mileage than the Ford diesels do pulling around the same truck.

The Excursion was 6.0 , it went to a 12 valve, mileage stayed pretty much the same. I also have a 2010 f250 that went from a 6.4 to a 12 valve, again fuel mileage stayed pretty much the same.

There’s no magic that will allow an 8k pound truck with the aerodynamic of a brick to get 25+ mpg if you drive it around normally. Maybe driving on flat ground, in low humidity, at 40 mph, with a tail wind would, but the Ford diesels would probably do pretty good too under those conditions.

There are many reasons one may want to Cummins swap a Ford diesel truck, but increasing fuel economy just isn’t one of them.
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my son has a 2004 Dodge Cummins, it gives him 21 to 25 mpg at 70 mph on level ground.

the factory sent out a recall notice to him, that his PCM had been flashed improperly, it did NOT meet California emissions...
he lived in Escondido, CA at the time.

he is a diesel mechanic and smelled a rotten apple, he started asking a LOT of questions and found 2 or 3 other 2004 Cummins owners who allowed the dealer to reflash their PCMs....

all of them lost their 20+ mpg economy and it dropped to 12 mpg

he told them to F off, it still gives him 20+ mpg
he now lives in Georgia, USA and to hell with emissions crap.

 

Well, they're not apples to apples comparisons. 6.0 to any cummins injection system, the advantage goes to the cummins, because HEUI suffers a loss of efficiency (using thick engine oil) that the rotary, common rail, or in-line pumps don't suffer from nearly as much. The "make up" from this trade off of mpg is an extreme increase in longevity of components. Plenty of HEUI engines go 300+k miles, with proper maintenance, and never have to touch an injection component. Even a diesel with an in-line pump will likely need injectors long before then. Common rail, different story. They are the most efficient (with no emissions), but injectors won't last nearly as long as HEUI or mechanical, and when they fail, they take cylinders with them.

6.4 to 12v getting the same mpg tells the story, comparing common rail to old technology and getting the same mpg makes my point. The 5.9 having only 6 cylinders and massive stroke (in comparison) to the 6.0 or the 6.4 means more hp at a lower rpm for the 6 cylinder. The most efficient engines on the planet are the ones that run in gigantic ships (approaching 50% thermal efficiency), and turn only a few hundred rpm. The 6.0 v8 configuration needs to rev way higher than the 5.9 with its 6 cylinders, this is no secret.

I wont get into what I've swapped and into what, but I am still genuinely curious what you get in your excursion with the 12v/nv4500 combo, which I doubt is 13 mpg.

 

Like I said above, mid-teens ( between 14-16) MPG is what the Excursion gets with a P-pump Cummins 12 valve and an NV4500 manual transmission.

With the original 6.0/ 5r110, it also got mid teens (14-16). Same tires, same gear ratio.
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Believe it or not, a 3rd gen dodge Cummins truck is actually a good bid lighter than an Excursion ( the Excursion is a little on the portley side, it’s even heavier than the f250 of the same years).

Third gen dodge guys realistically get high teens on the regular, 20+ is possible in those with the right driving habits. For me to get 20+ in the Excursion, I’d need to be driving downhill.
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Have you ever weighed your ex? My 02 is about 7800 lbs, not too heavy in todays standards, but heavy for the era. I wouldn't contemplate a swap unless the 6.0 was trash, but I was expecting more than 13mpg. I have a 7.3 van that gets 16-17 stock, I would expect the ex to be around the same weight as my '02 dodge with maybe a few hundred pounds extra.

 

According to the Google there is a min of 1500 pounds difference between a third gen dodge and a diesel excursion.

I’m currently at my in-laws house for Christmas. Drove up in the Excursion with four people, two dogs , all the Christmas stuff , basically the inside was pretty packed, but no trailer. After a four hour drive up here ( at 70-85mph) I refilled and hand calculated fuel mileage to be 15.6 mpg . I could probably get it to be a little better by driving slower, but this is Texas, any slower and I’d get ran over.
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(deleted the photos for the sake of the thread)

Can't rely on google for politics (Propaganda) or weights of vehicles. The range they provide is obviously for gassers. My truck weighs 7500+ lbs, been on the scales many times in the last 20 years. I don't think the ex is much different, but haven't had it long enough to scale it, it's the family wagon too so opportunities will be seldom, unless I make a trip specifically to weigh it.....which I'll eventually do.

I think your mpg is solid with full fam and driving for disregard of mpg. I'd hate to even see what my 13-14 turns into if I drive with total disregard for mpg.

 

According to the scale at the landfill, my second gen 97 dodge 4x4 diesel on 37 inch tires and with aftermarket 3/16 inch steel bumpers on both ends, weighted just under 7k pounds with me in it ( I’m about 260lbs ). Don’t know how accurate that scale is as it’s normally used to weigh garage trucks that are much heavier but it’s the only time I’ve ever weighed it without a trailer hooked to it.

That particular landfill weighs you on the way in, then again on the way out and charges you based on the difference. Since money is tied to the reading that scale gives I assume it’s decently accurate but of course they don’t exactly give you a certified weight ticket or anything.

The bottom line is an Excursion is heavier than a similarly equipped pickup truck and as such will always get a little less fuel economy. But really no one that’s overly concerned with fuel economy is driving an Excursion anyway. And I don’t see a realistic way to ever get into the 20+mpg range in mine (a lifted 4x4), maybe a lowered and lightened 2WD Excursion driving slow could get there.
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this mpg pix is right after 2 fillups,

1st from Price, UT to Tulsa

2nd from oologah to OKC and back to oologah topping off before and immediately when I got home.... run down and back empty, 50th high school reunion.


this is the mpg figures




this was being towed from price, UT to Oologah, OK which resulted in the 15.39 mpg




on this trip, the truck was bone stock, no addons at all.

NOTE: the Hydra Tuner does NOT improve MPG,
that is a fallacy that the manufacturers talk about, to get you to buy their products.

I drove 5300 miles with the Hydra Tuner on 65hp daily driver economy mode, and averaged 15+ mpg