Engine/Transmission (1998.5 - 2002) Mileage--has anyone really improved it?

[Diesel Freak]

Run the truck between 1600-2500. 1300 is way too slow!



do not BOMB the truck before you find out if you actually have a problem. It looks to me that you are not driving the truck efficiently.



I have never gotten over 19 MPG on my 2001 ETH.



I now average between 16-18 with DD3's a timing box, and a PDR40. My worst has been 13. It was a very fun 13 MPG!



I personally do not think any performance fueling modifications raise fuel milage..... the human tendency to use the extra power will negate any gains.

 

[geusterman]

Good point Lone Ram--35's made truck taller with more wind resistence. My 245's are stock narrow but slightly taller which moves the truck up.



Hohn. Thanks for thoughts but I want you to know that the mileage was identical stock, up to 10K when I started my "improvements". I love the lower GV rpm on the highway. I can even tow 10K with GV on flat and still have plenty acceleration. That is the EZ. Usually I tow in 6th however. I must get my guages in so I can watch egt's but water temp same as stock. In other words, I have not hurt my mpg with the mods. You are another advocate of DD2's which I must examine. George

 

[Groover]

ditch the GV

Guesterman, I agree with Hohn. If you can stand it just do a test ... leave the GV off and drive a steady 60 mph (not on interstate) for a trip and your mileage will go up.



1300 is grunting the engine and spitting out fuel that the engine can't burn in a lugging condition. I would want to keep the rpm's up to at least 1600 while cruising.



Heck, I have gotten 17 mpg on a 1500 mile trip at 2700 rpm.



3. 54's with GV and 19. 5's is way overkill on gear ratio even empty.



Please tell me you don't tow in double overdrive??



86 the GV, sell it to somebody with 4. 10 and stock tires that can use it.

 

[Diesel Freak]

Originally posted by geusterman

Bill Heath at Heath Diesel Performance (or whatever he is calling his biz these days in Ellensburg WA) makes a 400 hp mod with stock injectors and modded turbo

LMFAO :-laf :-laf :-laf :-laf

 

[jtisdale]

Another idea...

I try to keep my egt's around 600* or less (pre turbo), and my boost less than 5psi (stock turbo) at cruise (55-65mph). It takes a little practice to do this AND maintain speed, but it does help keep the mileage up. Of course, it may be impossible to do this running mountains, or while towing.

Lower egt and less boost = less fuel @ given speed = higher mileage. Oh, and dont use the cruise. The cruise control on my truck kills my mileage.

 

[Hohn]

Originally posted by geusterman

Good point Lone Ram--35's made truck taller with more wind resistence. My 245's are stock narrow but slightly taller which moves the truck up.



Hohn. Thanks for thoughts but I want you to know that the mileage was identical stock, up to 10K when I started my "improvements". I love the lower GV rpm on the highway. I can even tow 10K with GV on flat and still have plenty acceleration. That is the EZ. Usually I tow in 6th however. I must get my guages in so I can watch egt's but water temp same as stock. In other words, I have not hurt my mpg with the mods. You are another advocate of DD2's which I must examine. George

George:

If your STOCK mileage was that bad, then I would have some functional test done. I am glad you like the lower rpms-- your ENGINE might not, though. Many critical engine parts are splash-lubed, and lower rpm means that they often don't get enough lube. Moreover, it's harder on the engine, since it is doing more work per revolution. It seems illogical, but I bet that your Cummins would prefer to be over 2K forever than run the low RPM you are showing it. I believe the torque peak of 1400 rpm is the lowest I would go-- 1500 is probably even better.



You need gauges. You can run EGT high enough to liquify pistons before you would EVER notice it on water temp. I suspect that once you see boost, EGT, and Fuel pressure, you will have an idea what is wrong. I think it is a mistake to do ANY improvements before you put in a fuel pressure gauge-- if you have the money for the toys, you have it for the things you REALLY need. Spot checking FP really isn't enough IMHO.



Aerodynamic drag does not significantly change by having a higher truck. The frontal area of the truck (and the overall shape of the truck) is the biggest factor. Raising or lowering the truck doesn't change the frontal area. Frontal area is the area (sq ft) of the silhouette of the truck when viewed from the front.



Finally, the CTD has a rep of getting better mileage as the engine wears in the first 50K miles or so. you might just need a few more miles to see the higher numbers.



May I ask what pursuaded you to go with the 19. 5 wheels and the GV unit? Are you towing VERY heavy? you only NEED the large wheels if you are exceeding GVWR by quite a bit. That's another topic for another time. The GV is best used when you have a very short axle ratio (4. 56 and up) and need a little longer legs on the hwy. With a 3. 54, peak hp (2700 rpm) is at over 90mph! Do you tow faster than this? I bet the gear splits are nice anyway.



Please don't take this the wrong way. I have no desire to ridicule, defame, embarass, or insult you in any way. Most of my posts are designed specifically to:1) spread accurate info 2) save money 3) help others learn from my mistakes, and 4) entertain--- in that order. Sometimes i look smart, often i look stupid. but hey, I am really just hoping someone can get more enjoyment out of their CTD-equipped ram.



I personally don't see a need for either the GV or the 19. 5s, but then again, I have a pretty small budget, and that's all I am reponsible for



Oh, and the DD2s are worth every cent of their price and then some!!



HOHN

 

[Hohn]

Originally posted by Diesel Freak

LMFAO :-laf :-laf :-laf :-laf

I am pretty sure that I had over 500 rwhp with just an EZ, so have to be like 800 now that I have injectors, right?... ... or maybe not.



We ALL know that only a Powerstroke can clear 400 hp with just a modded turbo, right? This, of course is a product of Ford math-- the same math that shows a new 6. 0 with 320 hp stock

 

[jbone]

I agree about diagnostic--just got back from 1000 mile round trip and got 17. 5 mpg doing 65-70 (about 2200 rpm) most of trip!!!

That's with 4. 10's--probably something in that 'to low rpm' also... ... R, J. B.

 

[Wiredawg]

Higher RPMs is key

No, no real improvements in mileage with upgrades. I drive the truck pretty aggressively or I might experience some mileage improvements.



IMHO lowering the RPM on these 24V trucks below 1800 RPMs takes them out of their power range (1800 to 2500 RPM). My 3500 loves to run at 2100, but w/ 4. 10s I end up running 2400 quite often. Not worth the cost to me to change anything in the gear ratio 'cuz this bad boy really pulls my 17K lb 5th wheel.



I get 13 to 16 mpg running empty (10,100 lbs) and driving it like I stole it. BTW: My neighbor has a 99 3500 QC 2wd and has been bragging about getting nearly 22 mpg. I noticed his tank was at the half level and asked him how many miles on the trip meter. He said 301 miles. Well, my 2500 used to get 17 mpg showing 300 miles at the halfway mark. I told him and wasn't too happy.



Wiredawg

 

[OnlyCummins]

I'll echo the engine hasn't broken in yet & rpm is far too low sentiments. And, FWIW, I think some of the mileage claims should be taken with a grain (barrel more likely ) of salt. It seems very few respondents post speeds at which the mileage was obtained -- 55, 65, 70, or ... ? Or for that matter, trucks gearing. 20+mpg at 55mph does not impress me, OTOH the same mpg at 70mph does. With 4. 10's, even more so (sorta, kinda doubt it ). FWIW, my old '95 K3500 (K=4WD) 6. 5L TD HO (High Output) crewcab, longbed, with hi-rise topper, 235/85R16's, single rear wheels, 4. 10's & NV4500 got high 15'smpg to low 16'smpg (corrected for the 235/85's) traveling at 80mph (indicated, actual was ~83. ?) with rpms at ~2800.



IMO, wind resistance & gearing are the two big 'killers' of fuel mileage at higher speeds. Here's something from the thedieselpage that shows the relationship btwn gearing & fuel mileage (try to ignore that it's for a GM, principle is the same for all vehicles):

6. 5TD engine rpm (4L80E transmission) and typical fuel mileage at 65 mph:



3. 08 gearing 1690 rpm (23 mpg)



3. 42 gearing 1880 rpm (21 mpg)



3. 73 gearing 2050 rpm (17 mpg)



4. 10 gearing 2250 rpm (15 mpg)

(~ = approximate & is pronounced tillduh)

 

[Smokin Joes 2500]

fuel mileage

Originally posted by Hohn

I am pretty sure that I had over 500 rwhp with just an EZ, so have to be like 800 now that I have injectors, right?... ... or maybe not.



We ALL know that only a Powerstroke can clear 400 hp with just a modded turbo, right? This, of course is a product of Ford math-- the same math that shows a new 6. 0 with 320 hp stock

FORD SUCKS!!!!!! We do (with 6 cylinders) what they do with 8 (sometimes were better) I love my truck & wouldnt trade it for 12 powerjokes! Ill tell ya I got around 10mpg when I got my truck (84 miles on it) it now has 7200 and I usually get around 13-ish. Best Ive got has been 14. 8, I always hand figure and always fill tank to the brim! Look at my profile other than having 4:10s Ive got a big fuel hog. So far Im happy but hope mileage goes up as it gets more broken in! Hope you figure your prob out, I agree with the others though dont bomb until you are sure that is what it is! My truck doesnt do too much better in OD than it does with it off maybe 1mpg difference.

 

[geusterman]

Good stuff here. The 400hp by Heath Diesel is flywheel hp. Bill Heath was racing twin-turboed 6. 2's in mid 80's and is a master diesel tweaker.



The 19. 5/245 were part of a plan to carry a heavy camper without duallies. Have air-adjust Ranchos as well. The 245's are taller than 225's which are recommended as they are similar to stock in overall height but 245's carry almost 5000 lbs apiece and have 16 ply rock-resistant sides.



The GV (even with 3:54's was a plan to "reproduce" the delight of my 83 6. 2 Blazer which had either 3:08's or 3:42's and was a long-legged 21-23 mpg sweetheart to drive on long distance. Now, I could get by without my GV. In fact, if it really affects exhaust gas temps negatively then it has no value other than a quiet ride (I am willing to do a noise nazi on the truck if I can cure the mileage issue--no sense dumping more long-term dollars in a rig running 6 mpg less than it should).



I will get my guages. I need a nice 3 pod with Egt, Boost and fuel pressure. Have not seen a nice cluster yet. Anybody????



The main thing here is that mileage has ALWAYS been about 14 mpg with or without mods. Those mods shoulda had some effect. I need to know if there is something a stock truck can adjust to improve mileage. George

 

[geusterman]

Smokin Joe. Looks like the same guy set up our 2002 engines on the assembly line. George

 

[OnlyCummins]

Just as a reminder, I think, break-in is considered complete at roughly btwn 10K & 20K miles so both of you still need to put a few miles on your trucks before they can be considered fully broken in. Another reminder, I believe, above estimates go out the window if synthetic engine oil is used before the break-in is complete. Will take "forever and a day" with synthetic .



Good luck to both of you.

 

[Hohn]

Only Cummins raises some good points. I also happen to believe that wind resistance and topopgraphy are the biggest factors in hwy fuel economy. Tire rolling resistance goes up as you go faster as well.



Aerodynamic drag is exponential-- if you go twice as fast, you have 4 times more drag. Other forms of drag are exponential or close to it as well. VERY few things when it comes to total load on the engine are linear-- the grade you are pulling is about the only thing I can think of.



The info on gear ratios and fuel economy is misleading. While it MAY be true for a gas engine, such is not the case for a diesel. The is because gassers have a mostly fixed air/fuel ratio, while diesels vary that ratio in proportion to load. That's why a diesel will get VERY high mpg going down hill. The the only way to comparable mileage out of a gasser downhill would be to turn it off and restart when you got to the bottom! A gas engine just can't run as lean as a diesel can.



The significance of the above paragraph is that fuel economy for a diesel is detemined primarily by load on the engine and how well it is extracted to get work. Think of all the factors that determine this on the hwy: aerodynamic drag, tire rolling resistance, incline of road, transmission gearing, axle gearing, etc. These are just a few.



The laws of physics tell us that two trucks with different axles (3. 54 and 4. 10) which are otherwise identical swill get the same mileage if they are both within the reasonable RPM range of the engine. At 60MPH, a 3. 54 is 1600ish rpm, while the 4. 10 is about 2200. Even with different RPMs, fuel economy will be the same. This is because while they 4. 10 truck is turning more rpm, it is burning less fuel each time a cylinder fires because the engine is under less load with the higher axle ratio. They cancel each other out.



Ask yourself, "what determines engine RPM in a diesel"? Answer: fueling rate, or how much fuel is injected per combustion event per cylinder. With no throttle, this is how diesel engine speed is controlled. When you increase fueling to a certain point, RPM will rise until engine output equals the load on the engine. That's why the same amount of pedal that gives you 2500 rpm in second will not give you the same rpm in 5th! The higher gear (5th) placed more load on the engine, which means you need more fueling to generate the same rpm level.



**Remember: in a gas engine, higher rpm causes you to use more fuel. In a diesel engine, using more fuel causes you to have higher rpm!!**



Think also in terms of engine "resistance"-- a combination of the engine's internal resistance to rpm and the external resistance to generating rpm caused by the external loads placed upon it. The power generated by a given fueling rate has to overcome this to generate rpm.



It's similar to why you can't gernate much boost in lower gears. In 1st or second, the engine revs fast because there isn't much load on it, and the rising revs swallow air as fast as the turbo can make it. in high gear, the engine has a LOT of load on it, and can't increase rom easily. Thus, boost builds because the turbo gets WAY ahead of the engine.



Sorry to get so techie, but all this is to say that max fuel economy comes from minmizing load on the engine while running in the highest gear within the operating RPM of the engine. Try a hwy tank running 55 on level ground- my eth/dee will get almost 24 mpg doing that. Since I usually run closer to 75 in moderate hills, upper 19s is more what I usually get.



Other fuel economy pointers:

- tire pressure

- slow down

- cruise control-- don't use the "accel" button on the steering wheel!

- synthetics in driveline (axle, trans, t-case, engine, etc. )

- use a fuel additive



HOHN

 

[RustyJC]

Originally posted by Hohn

The laws of physics tell us that two trucks with different axles (3. 54 and 4. 10) which are otherwise identical swill get the same mileage if they are both within the reasonable RPM range of the engine. At 60MPH, a 3. 54 is 1600ish rpm, while the 4. 10 is about 2200.

With my 4. 10's:



2000 RPM = 60 MPH

2350 RPM = 70 MPH



Our truck was ordered strictly to tow our 13,500 lb 5ver (the 4. 10's provide a 21,500 GCWR vs 20,000 GCWR for 3. 54's. ) For our towing application, the 4. 10's are great - I wouldn't consider anything else! For running empty, they stink! Different horses for different courses - pick the right tool for the job.



We just made a round trip towing our 5ver from the Houston area to Nashville, TN and back. Towing in Tennessee (70 MPH truck limit and some pretty good hills on I-40 from Jackson to Nashville), we ran 72-75 MPH uphill and down in 6th gear and got 10. 5 MPG. In Arkansas (65 MPH truck limit and somewhat flatter terrain across I-40 and I-30), we ran 65-67 MPH and got 12 MPG. Aerodynamic drag and terrain obviously have a lot to do with mileage. I can't comment on empty mileage - our truck has never had a full tank of fuel run through it on the highway when it wasn't towing. I've seen around 16 MPG in Houston suburban traffic, however.



Rusty

 

[Hohn]

I was led to believe that the difference between the 3. 54 and the 4. 10s was about 600 rpm. . Turns out it's more like 300 rpm. Thanks for the good info, Rusty-- another great post.



My truck turns 2K rpm @ 70.



I can see why they went with the 3. 73 in the new trucks. The extra rpm won't hurt the engine or economy any, and gives a nice little boost to the grunt.



HOHN

 

[RustyJC]

Just in case anyone wants to compare gear ratios & RPM:



3. 54 = 2000 RPM @ 70 MPH



4. 10 = 2000 RPM x (4. 10/3. 54) @ 70 MPH, or 2316 RPM.



OK, so 70 MPH isn't 2350 RPM with the 4. 10's! Blame it on parallax error, 125 RPM markings on the tach and a severely nearsighted old man. But, then again, the tires are taller on a 2500.....



Also, aren't the 3. 73's there to offset the taller tires on the 2003's in addition to capitalizing on the higher operating speed range of the HPCR Cummins?



Rusty

 

[OnlyCummins]

Originally posted by Hohn

Only Cummins raises some good points. I also happen to believe that wind resistance and topopgraphy are the biggest factors in hwy fuel economy. Tire rolling resistance goes up as you go faster as well.



Aerodynamic drag is exponential-- if you go twice as fast, you have 4 times more drag. Other forms of drag are exponential or close to it as well. VERY few things when it comes to total load on the engine are linear-- the grade you are pulling is about the only thing I can think of.

To further expand on the relative impacts of rolling & air resistance I'll offer up examples from bicycling. Principles are the same regardless of vehicle. I used to be a quite competitive cyclist & consequently did quite a bit of research into the relative impacts of resistive forces on a bicycle & rider. From one of my references:



RR=Rolling Resistance & AR=Air Resistance

10-kph (~ 6mph) -- RR~80% of total resistance to movement & AR<20%

20-kph (~12mph) -- RR~46% of total resistance to movement & AR~54%

40-kph (~25mph) -- RR~18% of total resistance to movement & AR~82%



To put the #'s in perspective, 25mph would be considered to be a maximal effort for the avg recreational rider but just "motoring" along for a world class athlete. Further perspective: The top speed record, unassisted (without a wind shield/screen), is ~50mph, while the top speed record, assisted (behind a huge wind shield/screen mounted to a funny? car) is around 150mph. That's why there's such an emphasis on aerodynamics in an event called time trialing (basically individual rider against the clock). Also, the exponentially higher wind resistance at higher speeds is why 'drafting' (getting behind someone's slipstream) is of such importance in groups of riders. In groups, drafting plays a big role in energy conservation and that's why a group of 2 or more riders riding in a paceline (single file) can maintain much higher avg speeds than an individual cyclist.

The info on gear ratios and fuel economy is misleading.

Don't know if you're referring the thedieselpage example I listed but if you are, the info listed is for a GM diesel.

While it MAY be true for a gas engine, such is not the case for a diesel. The is because gassers have a mostly fixed air/fuel ratio, while diesels vary that ratio in proportion to load. That's why a diesel will get VERY high mpg going down hill. The the only way to comparable mileage out of a gasser downhill would be to turn it off and restart when you got to the bottom! A gas engine just can't run as lean as a diesel can.

FWIW, my old gas-powered '94 Chrysler T&C would max out the instantaneous fuel mileage display on downhills. 99mpg -- woohoo !!!



... snip ...

The laws of physics tell us that two trucks with different axles (3. 54 and 4. 10) which are otherwise identical swill get the same mileage if they are both within the reasonable RPM range of the engine. At 60MPH, a 3. 54 is 1600ish rpm, while the 4. 10 is about 2200. Even with different RPMs, fuel economy will be the same. This is because while they 4. 10 truck is turning more rpm, it is burning less fuel each time a cylinder fires because the engine is under less load with the higher axle ratio. They cancel each other out.

The key qualifier is "... within a reasonable RPM range ... ". From all I've read, the fuel mileage 'sweetspot' is attained at, or just slightly above, the torque peak of an engine. Assuming an 1800rpm torque peak, I'd guess the 'acceptable' range for peak fuel mileage would be 1800-2000rpm. Using the formula below & solving for speed & assuming 4. 10's & 245/75R16's then the maximum speed



RPM = (336 x Overdrive Ratio x Rear Gear Ratio x Speed) / Tire Diameter



SPD = (RPM x TD) / (336 x ODR x RGR)

SPD = (2000 x 30") / (336 x 0. 75 x 4. 10)

SPD = ~58mph



for best fuel mileage would be ~58mph. Obviously not a speed at which most of us drive on the hiway/freeway. BTW the formula above applies to all vehicles regardless of motive power. As a check of the formula's #'s, the rpm on both my gas-powered '95 K2500 Suburban & my previous diesel-powered '95 K3500 are/were both in the area of 2800rpm at ~80mph. So ... inputting the appropriate #'s into the formula:



RPM = (336 x 0. 75 x 4. 10 x 80mph) / 30" = ~2755



The only difference btwn the 2 vehicles in stock form aside from one being gas-powered & the other diesel-powered is in transmissions. The Sub has the 4L80E automatic, while the K3500 has the NV4500 but AFAIK both ODRs are the same so that's a nonissue.



... big snip ...

HOHN

 

[OnlyCummins]

RustyJC, you can also use the formula I listed above to get approximate rpm #'s. The # I used for tire diameter was nominal & may not be the actual, as mounted, TD. #'s aren't that far off tho:



With my 4. 10's:



2000 RPM = 60 MPH vs ~2066rpm from formula

2350 RPM = 70 MPH vs ~2411rpm from formula