the REAL facts about fuel "economy" on the CTD trucks!!

[Diesel Nut]

I am just wondering, has anyone taken into consideration that class 8 trucks run engines with an operating range half that of our trucks? I mean, yeah, these are some effiecent trucks, and so are over the road trucks. But when Icruise at 75 and 2100 rpms I am at the governed speed of an ISX. Go figure. RPMs = fuel usage. Doesn't matter how good your engine is at using fuel, you will have more to vercome beause the engine has to run faster. I see this being true as when I put 265 70R19. 5's (34. 5") tires on my 03 and then recalibrated the speedo (within 2mph @65 based on gps) I picked up like 1. 5 mpg. Botom line is the slower you can spin em and not heat em up or bog em down the more fuel mileage. What do you think rigs are doing at 60 mph? 1200, 1400 rpms? Make sense. Wana check it out? Run you truck in OD at 1400 rpms and see what your numbers are. I bet if ou are honset with yourself and us you will be 25+ mpg. Just thinking out loud. . . .

 

[steved]

I don't know why we are comparing apples to oranges... yes both are diesels, but they are designed for two completely different purposes.



steved

 

[BBowers]

They are not completely different. A dodge p/u is simply a scaled down rig with a more compliant coil up front suspension. The engines are given a 3400 rpm redline to give us more car like performance when demanded, but who redlines their ctd anyway in normal driving? The ISX would likely hold together if spun to 3400 occasionally, but they don't need to. It would also affect the available warranty duration. The K series 19 liter cummins was spin tested to 3500 rpm during development without failure to prove its durability. 19 liters!!! I use a max of about 2600 rpm when i am driving on the harder side, which is rare. No, i have never seen over 22 mpg on my overhead at 45 mph in top gear, converter locked, achieved by slowing down from about 50 till the converter is about to unlock or the transmission is getting ready to downshift to third, I don't know which. I think It was turning about 1200 rpm. So, I guess If i was going 65 at 1200 I would see 30 mpg on level ground posssibly, but without switching to 3:42 gears and tall tall tires and maybe a GV OD unit, I would never achieve that. Once again, I don't see why we don't have that gear choice. THe gas trucks with 3:50 ish gears often turn about 1800 rpm at 75 mph, so why can't our torque monster dodge ctd's have the same or taller final drive ratios to take advantage of that 610 ft lbs at 1600 rpm when towong light or empty? Tow/haul mode could cancel out the top gear and so forth, just like on the gas trucks. I guess maybe the oil companies don't want us all to see the max potential of diesel so they buid a friggin gas truck with a diesel dumped under the hood. I had a 2000 expedition with a 5. 4 260 horse v8. It produced 350 lb ft of torque "at a low 2500 rpm". It had 3:31 gears, weighed 5500 lbs, and got about 18. 8 mpg at 74 mph. It had a trailer towing rating of 8500 pounds too, but anyway, point being, our trucks ought to have much better mileage empty.

 

[steved]

But, still... one is designed more or less as a car, the other to tow greater than 80k.



You cannot "simply" compare them... no matter how you look at it.



steved

 

[AGrubbs]

Well, riddle me this one then. . This is the deal. .



I get about 21-22 real MPGs out of my '03 4x4 2500 in overall driving, no towing, running 55-65MPH over the past 10K miles (straight #2 fuel).



I get about 48-54 MPG out of my '99 Jetta TDI on hiway driving same speeds.



Truck is about 7200lbs, Car is only 23-2400lbs. Truck is rated 305 HP and car is 90 HP. Both are manual transmission. 1/3 the HP and 1/3 the weight.

I don't get 65 -70 MPG (3 x truck)in the car which by using the logic at the start of this thread I should be getting. . I only get about double the MPG's.



I think you are doing an apple to oranges type of comparison. . it just is not the same...

 

[Hohn]

. I just read a Cummins broshure on an 02 ISX that was averaging 8. 6 MPG and had 580k miles. My 05 2500 4wd auto is only getting around 14 now, more like 13 and I am pretty dissapointed right now. Dealer says it is fuel 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.

Sounds like you should re-power your Dodge with an ISX.


I'd love to build a street rod with long hood and a Class 8 engine under it.

If you geared it really tall (say 1:1 differential), you could probably get good MPG from it. If course, you'd also be idling at 65mph.

 

[steved]

I'm still researching 3. 42 gears for my truck right now... seems they make it for the front axle, but not the rear as of yet.



steved

 

[Gray 5.9]

I also went to AAM's website and found that they have lower gears for our trucks, gotta be cheaper than a US gear overdrive.

 

[steved]

I also went to AAM's website and found that they have lower gears for our trucks, gotta be cheaper than a US gear overdrive.

What they list and what they actually have are two different things... they only produce down to 3. 73 at this time... but the site (and their books) list down to the 2. 50 range.



steved

 

[sled4fun]

On the subject of installing a U. S. Gear. I had a 1976 chevy van with 4. 11 gears and a U. S. gear 22% overdrive. Drove from Sacramento to L. A. at night in the summer (No a/c on) with it in overdrive. Ran 65 mph at about 2800 rpm and got 13 MPG. Came back in the daytime and ran it out of overdrive with the A/C on and got 13. 5 mpg. On that vehicle it it was not worth hearing the whinning noise of the U. S. gear unit.

 

[BillGotthelf]

Interesting posts. What I have found to increase mileage in our trucks: A timing box set on a lower setting ( like TST on 3/3) will give 1. 5 to 2 more mpg's. With an automatic a lower stall (1700 rpm) TC will increase your fuel mileage around town by 1 or 2 but not help on the hi way. Injectors don't seem to help in 3rd gens, neither does a larger exhaust. You get about . 5 more mpg with the 3. 73 rear over the 4. 11. Oh and my 99 with 700+ hp got better hiway mileage than my 04-go figure.

 

[RMarkey]

Despite comparing trucks, we should still be getting better mpg.



I thought the optimum speed for aerodynamics was 71. 4 mph anyway!

On the hwy for the most part the slower I go the better I get. On mostly flat ground and closer to sea level if I run around at a constant 55 mph I can get the over head to read maybe 18 for a while. Then again it's usually averaging about 16/16. 5 for those conditions, and I'm tickled pink. But I'm usually at higher altitude and nowhere near flat. I run the mountains usually and have somehow managed to get almost 16mpg at speeds around 85/90mph! It very seldom happens but always makes me wonder why I can get the same results driving completely differently. Except for being illegal, Id rather drive the 85/90mph!



What gives with that?

 

[Diesel Power]

i dont know if you have the patence to drive 55-60mph but i do and i see any where from 22-25 mpg depends on traffic, you'd be surprised how much better mpg is when you slow down. the best i ever got was last year before they went to winter fuel, 27. 6 mpg hand calculated! 55mph all cruise,it works when i need it to----but i hate going so slow. if i drive like i want to i'll get 17-19 on the highway 70-80mph :-laf

 

[WMoroni]

I want to know why all the old 90 model 12 valve truck I had got 20-25 easy and my 05 gets 14mpg. Both trucks are set up basically the same, expect for 3. 55s verse 3. 73s and an extra gear in the transmission. Best the 05 ever got was about 19 but that was one time on one trip and never again. The guy driving my old truck says it still gets about 21 every tank and its only got about 400,000 miles on it. Seems every time the epa wants cleaner air the mpg goes down. Doesn't make sense. How can burning 14 gal per mile be more enviromentaly friendly than burning 25 gal per mile? Shouldn't more fuel being used make more pollutants?

 

[ROGER/DODGER]

I have been watching these posts for about as long as there has been controversy. it seems like the 04. 5s got the best milage and then it started dropping in the 05s and then the bottom fell out in the 06s ! I believe it has a lot to do with the staged injection cycle and lack of timing advance on the 06s. Probably mandated by tighter EPA regs.

Like someone else said here, they are making them run cleaner with the cats and burning a lot of fuel on the late injection pulse. I'm no engineer, but this looks like a possibility to what happened to the mileage !

 

[mlewis73]

Thanks for ONE person in TEN getting the point. the RIgs these days have LOW end ratings of 425 hp, and often are rated at over 500!!!! Yes, this is delivered at lower RPM, but not THAT low. 2100 is the lowest I have heard of on an old KTA19 1150 cid 19 liter cummins that typically developed 450 to 700 hp!! They went by the wayside in trucks as to emissions compliance problems but what great engines! Anyway, they are doing more with less these days with displacement, but I am sticking to my guns. WHERE IS THE FUEL MILEAGE? Also, I DO USE MY CRUISE CONTROL as often as possible. If I am on a stretch of road with no traffic lights, on goes the CC!! I try and play with the throttle to get the transmission to upshift early and keep the revs down. I try to get the transmission to shift up and lock up the converter as soon as possible to make use of the 600 plus lb ft of torque from the 5. 9. What is truly ridiculous is I have had guys respond to my query on economy in the past with claims of 19. 5 mpg with piers diesel street twin turbos making 600 hp!!! same truck as mine! Oh, and one last thing, I forgot to mention the extra 12 or so inches of vehicle WIDTH on the semi vs. dodge ctd comparo. Major difference in wind drag!!! So what gives? Are they purposely designed to eat fuel to keep us ignorant of how diesel could wean us off of expensive to produce gasoline? God forbid we all drive diesels!!

You're absolutely right.



I think it has everything to do with the EPA making these trucks poop air out the tailpipe that is CLEANER than the air that goes into the intake.



Okay, maybe not that bad, but still. Emissions laws killed your fuel mileage. You can thank the EPA for that. God help those who use these trucks for business... almost makes more sense to buy a 12V, rebuild it and tune it for power. I get 18mpg with my '91 Ford V-8 diesel on the highway. VEE EIGHT!! 13 pulling a 10,000 pound horse trailer. There is no excuse for the newer trucks to not deliver better mileage than that, and my mileage was at 75-78 miles an hour. Not crawling at 60-65.

 

[steved]

To be quite honest, the attractiveness of the diesel is sorta going away because 90% of us bought them for the fuel mileage.



I'll still own one because I average 65k a year... need something that will hold together for more than a couple years... but I have bought my last NEW truck.



steved

 

[Gray 5.9]

BillGotthelf,

What rpm's and MPH are you running to get the best economy with TST on 3/3 setting.

I'm strongly considering the TST PMCR as my next addition for more power as soon as my warranty runs out. Being that I drive 660 miles per week to work, I'm running mostly HWY at around 70 getting (hand calculated) 16. 7 mpg empty.

 

[AKaiser]

18. 9 empty hand-calc'd at 70-75 open interstate, easy. Got closer to 17. 5 pushing 80-85, same road. Over 20 on 2 lanes, 50-55 average.

 

[MCummings]

Now I'm very upset.



Me Father in Laws Ford Ranger only gets 18MPG. WTH? That's a tiny truck. Should get 50+MPG...





Merrick