I am one of the lucky ones with excellent fuel economy--typically 20+ running empty and and mostly highway. For those doubters of my math skills, I do know my gazintas. (two gazinta four two times)
The big issue is why? Many have asked for help, and on this forum we typically give the same old answers, assuming that each CTD is exactly the same out of the factory. There must be more to it. What are other variables other than rpm's, speed, heavy foot, etc. Are there mechanics out there that have some insight into this?

 

IMHO, and I don't care about offending anyone, is that poor mileage woes are usually driver related. I'm with you SWG, I've always gotten good mileage with my truck. I've towed with it since the day I got it. Nothing heavy like some guys on here, but I tow 5k most everyday of the week going from job to job. When I read most posts on here saying that, "I drive like a Granny", I usually take it with a grain of salt. I do drive like a Granny, both towing and empty, because I almost always have all the other cars passing me both city and highway!! The best I've gotten is 21.2mpg hand calculated on a 500 mile round trip running empty at 65mph!!

 

Tire pressure is a big one, missed astoundingly often in these discussions. Front tire pressure is ABSOLUTELY CRUCIAL to mileage-as in, you must have enough of it. I run 80 psi in my diesels up front.

Another thing that probably plays a role in mileage differences is slight manufacturing tolerances in the crank position sensor pickup and tone wheel assembly. I've seen at least one or two people comment on how they did the tone ring mod, and in the process discovered that there was a bit of slack in there from the factory. I can't remember exactly where the slack was, but it was enough to make a degree or two of difference in timing from one truck to another. That's enough to account for noticeable differences in mileage.

I don't know if this last thing is common at all, but it is also possible for one or more injectors to be bad in an insidious way, in this case meaning that the truck seems to drive fine, but just never gets good mileage. It does not have to result in fuel being dumped into the crankcase or billowing clouds of white smoke. Unfortunately, the dealerships will probably not do much if any injector testing based on vague complaints of poor mileage. It's hard for me to blame them too-they would be doing nothing but testing perfectly good injectors because people with low tire pressure and poor driving habits were complaining about low mileage. The downside to that is there are probably bad injectors running around out there that haven't acted up in a specific, easily identifiable way, costing their owners money on wasted fuel.

 

I with you guys, running 19-21 MPG at 65 MPH but at 70-75 it drops to 17-18. My tire pressure is 55-60psi both front and back with the stock tires and 65k miles.

 

At 65, I can get a little over 20. However, if I'm moving at 80, it's more like 16.6. It all depends on how you drive it. Most don't have the patience to accelerate up to 50mph while keeping the RPMs under 1800. Mixed driving I can keep mine at 17.5.

 

Go on and run the fronts up to 80-see what happens.

 

I am sure there is more to it. Currently I am at about 9000 miles on my 2006. It is an automatic, 4x4, 373 gears. I average about 18-19 on the highway, but at first it was maybe 14-15. The biggest improvement I saw was after I made several long hauls when I moved 400 miles across state and when I move my horse around from barn to barn, etc. I also pull my bass boat about 75% of the time. Now, when I drive empty or not towing, not only is mileage better, but the truck runs much, much better. Now, if I had a standard transmission, then my mileage would be even better, for obvious reasons. This is what most people dont ever realize when they compare one truck to the next, especially on the internet. A standard will be better then an auto, a 4x2 will be slightly better then a 4x4. It confuses me when people get on here and complain about fuel economy. I mean, you bought one of the largest vehicles on the market, what did you expect? The other thing to consider is how you work your truck. A guy who tows good weight and makes long trips is going to see that better MPG long before the guy who drives 15 miles to work everyday or tows a light load once a month. There are lots of variables you can attribute, but overall, you have to break the engine in. This means making it work, making it pull, haul, run for hundreds of miles nonstop.

 

Must be nice... When I drive like granny, I see MAYBE 17. Wierd... On a long hwy trip, my milage is good at first, then it get's worse, using the cruise and flat road. It can go from 17.5 to 14, and I pay attention to the way that I drive. I know it has something to do with my low gear rear end and bigger tires, but you guys are saying 20-22, there seems to be something wrong???

 

Have you recalibrated your speedometer for bigger tires?

 

Last week I drove 374.6 miles, mixed about 50/50 city-highway 25.004 gal=14.98 mpg However, on the trip back home, 98% interstate I made 22.34 mpg.....Cruise set at 2000rpm.....I drive on the tach, not the speedo...If it matters 2K is 73mph


BTW I run 32oz of PS White/MMO mixed 50/50 per tank....I see 1.5-2 better with this than with straight fuel

 

V8440 hit the nail on the head, air pressure is everything. the cummins weighs over a thousand pounds. thats a heavy front end. keep those tires at max. last summer i made it from portland to shasta lake,(in cali) and back to medford on 1 tank, 658 miles with a couple gallons left,hand calced to about 20ish even with the 4.10s cruising at 65. I run my tire pressure a couple pounds above max. thats a huge key me thinks.

 

My truck is not even near break-in, and I agree with these figures. I know it is not possible to drive at 50mph long distance in most situations, but I have tried it a few times when there's little traffic, late at night etc. Mileage at that speed on my truck(s) has been incredible, and I mean 25+ US mpg. I seem to remember the "gas crisis" of the 70's when the speed limits were reduced and enforced at 50 mph in a lot of places. Imagine the savings on fuel world wide if these speeds were possible??

 

my truck was always a constant 16 empty running to an from work since the new lift pump i just filled up for the first time since and i gain about 2.5ish to 3mpg i averaged 18.5mpg an that was one load of hay so now ill see what i get not towing any thing for this tank. I figured out also if you fill up at the line just under the half mark it don't hurt the check books feelings as bad

 

that beat you up bad on ride? roads in SC are "historic" - in other words, about the same conditions as Columbus found them, when he came over here looking for a CTD

roads here about beat your teeth out your head.

 

My 3rd Gen justs gets bad mileage compared to my 96' 12v 5spd. My 96' had the same power on the dyno as my 06'. I didn't care how I drove the 96'. The 06' is driven very nicely at slightly slower speeds paying attention to boost.

96' = 19-20mpg
06' = 17.5-18mpg

Combo of heavier vehicle, different gearing and emissions.