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

[RustyJC]

Originally posted by 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

I'm running LT235/85R-16E's for which Michelin lists a diameter of 32. 0" and 655 revs/mile. The LT245/75R-16E you used, again per Michelin, has a diameter of 30. 6" and 676 revs/mile. Therefore, I would expect my RPM at 60 and 70 MPH to be slightly lower than the formula values.



Rusty

 

[WadePatton]

~ cows come home

Basics. I say you need an EGT gauge no matter what. Every other gauge/reading/feeling/guess is secondary to that.



FWIW-my 4x2 y2k with 3. 54's and 265's running #2 with additive and Rosmella in the case through a 5600 with fresh oil and synthetic in the rearend-normally gets in the 18's under my lead foot--all conditions.



I did a mileage experiment the other day on the way to FL and drove 100 miles like an old mother goose, barely cracking 70mph on the interstate just to see if I could rack up a big number. Guess what?-- 22. 3! Fantastic. Now I know my truck can do it. _I_ on the other had, just cain't live like that. Oo.



Haven't calculated the rest of the trip yet, but I did embarrass some ricers and a GT mustang.





Looking forward to the mileage gains I'll get from going synthetic, adding big injectors and big turbo. Might take a while for it to pay off.

 

[OnlyCummins]

Originally posted by RustyJC

I'm running LT235/85R-16E's for which Michelin lists a diameter of 32. 0" and 655 revs/mile. The LT245/75R-16E you used, again per Michelin, has a diameter of 30. 6" and 676 revs/mile. Therefore, I would expect my RPM at 60 and 70 MPH to be slightly lower than the formula values.



Rusty

True ... but the "Overall Diameters" listed are nominal & manufacturers are allowed, I think, up to ~10% variance from nominal to actual. For example, just did some quick research & from a printout of Michelin tire specs I have for the XPS Rib LT235/85R16E the overall diameter listed is 32. 2" but the "Loaded Radius" listed for the same tire is 14. 8" which implies an overall diameter of ~29. 6" which is why my calculations produced #'s quite close to your posted #'s. FWIW, if I input the nominal diameter of 32" into the formula the results are:



2000 RPM = 60 MPH vs ~2066rpm (30") vs ~1937rpm (32") from formula

2350 RPM = 70 MPH vs ~2411rpm (30") vs ~2260rpm (32") from formula

 

[Hohn]

O. C. -- good post, lots of great info. Not sure what the snip means. .



The dieselpage examples posted are misleading, whether gas or diesel. They would lead one to believe that you could get 30 mpg out of our trucks by simply swapping in a 3. 08 axle. My point is simply this: taller axle ratios are not a quick ticket to mpg. Furthermore, this is true because of the diesel's varying fueling in response to load. A comparable gasser truck WOULD see higher mileage from taller gears, at least far more so than a diesel would.



I am not familiar w/ the GM diesels (other than their tendency to explode when installed in late-70s cars), so I am limiting my comments to the CTD.



Read above on George, who installed a GV overdrive w/ 3. 54s. no magic fuel mileage here from taller ratios.



The "magic rpm" band of these engines is not so narrow, but rather from peak torque to peak hp. In stock form, that 1400-2700 rpm.



Most gasser cars will max out a little fuel dummy on the downhill. But so would a diesel so equipped, and if you COULD get actual fuel consumption data at that point, you would see the diesel using much less fuel on the downhill. Again, diesels can run MUCH leaner than gassers.



So, to minimize drag and operate within the engine's power band, optimum speed for my truck (3. 54, 265/75, dee) is:

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

SPD = (1400*31")/ (336 x . 73 x 3. 54)

SPD = (43400)/ (868. 2912)

Spd = 49. 98 mph



Hohn

 

[RustyJC]

That's why, unless I have an accurate number for loaded radius, I prefer to utilize the revolutions/mile which Michelin measures @ 45 MPH for my calculations. Backing into an effective loaded radius from the 655 revs/mile figure, 5280'/655 = 8. 061' circumference, or a loaded radius of 8. 061'/2pi = 1. 2829' = 15. 396". This, of course, is less than the 32"/2 = 16" one might "thumbnail" for the LT235/85R-16E tire.



If you'll go back and look at my original formula, however, I was merely advising how to quantify the effect on RPM of a final drive gear ratio change (we were discussing 3. 54 and 4. 10 ratios) with no other changes. Thus, the loaded radius of the tire (a constant, in this case) doesn't even enter into consideration.



Rusty

 

[geusterman]

After watching mpg posts for the last year it seems that identical trucks can vary 3 to 7 or more mpg. The VW TDI's are real "dead one" with their 45 mpg rating and you can "milk" out more with good tactics like slower speeds, coasting, tire pressure, steady foot etc. BUT there is a real difference in CTD's. What I am trying to find out is: Are there adjustments that can be made to a stock 24V CTD to make mileage improvements????? George

 

[Hohn]

George, I think the only thing you can really do it try to make sure your engine is in tip-top shape. AFAIK, most people DO report better mpg with the RV275 injectors, and I think these are the ONLY injectors that have consistently reported mileage gains.



Bumping up the timing may help mileage a little bit, but tdr members report different results with this-- some see no increases, others see modest gains. To bump timing on a '02 HO, you need a box of some kind.



I would recommend the Diesel Dynamics version of the Edge EZ. They call it the True Torque Power Module. Most users say that they have the timing perfect on that box, and almost all report a gain. I have the EZ, which is perhaps a little too aggressive on the timing as they went more for torque gains than for mileage gains.



To answer you directly, there are no adjustments that I am aware of that will increase the mileage of a stock truck. Stock trucks are limited to mostly driving technique, fuel additive, and maybe synthetic fluids as options... .



HTH





HOHN

 

[ben_ak]

Just for comparison sake:



2000 4x4 Automatic w/3. 54 & 33k Miles (Edge EZ, 28 PSI, K&N)



Highway 19. 5 MPG

(Avg. Speed approx. 70 MPH, passing @ over 100 MPH sometimes)



Mixed Highway/City 14-17 MPG



All City 13-15

(Heavy foot syndrome)





I want to break 20 MPG on the highway but it's tourist season up here in Alaska and I can't stand sitting behind motor homes doing 40 MPH in a 65 zone.



I am going to do some testing with tire pressure and see what it does.

 

[OnlyCummins]

George, this might cheer you up:

Originally posted by DKupfrian

I was wondering the same thing. When I bought my truck new in 2000, I kept all fuel receipts and calculated by hand my milage. Let me tell you that at 60,000 miles my milage is still increasing. These trucks are not fully broken in until you hand the keys over to your grandson!



BTW

first year averaged 16. 5

second year averaged 17. 5

third year averaged 18. 5

forth year looking like it will be 19-20

4x4 with automatic



enjoy

Came across it on this forum but on the "V10 vs I-6 MPG Wishful Thinking" thread.

 

[geusterman]

Ha! Thanks OnlyCummins. I guess they "adjust" themselves over time! Hohn, you brought up some good points on the timing of the box and injectors. I was hoping that I could put it on the computer and make some adjustments at Cummins or the dealer. George

 

[OnlyCummins]

Originally posted by RustyJC

That's why, unless I have an accurate number for loaded radius, I prefer to utilize the revolutions/mile which Michelin measures @ 45 MPH for my calculations. Backing into an effective loaded radius from the 655 revs/mile figure, 5280'/655 = 8. 061' circumference, or a loaded radius of 8. 061'/2pi = 1. 2829' = 15. 396". This, of course, is less than the 32"/2 = 16" one might "thumbnail" for the LT235/85R-16E tire.

The spec sheet/tire I cited earlier also has the same 655 revs/mile figure so there appears to be a 'disconnect' btwn the two "effective load radius" & "Loaded Radius" #'s: 15. 396" vs 14. 8". Don't know how to account for it except for perhaps as the difference btwn nominal & actual tire diameters.

If you'll go back and look at my original formula, however, I was merely advising how to quantify the effect on RPM of a final drive gear ratio change (we were discussing 3. 54 and 4. 10 ratios) with no other changes. Thus, the loaded radius of the tire (a constant, in this case) doesn't even enter into consideration.

Very true ... even an incorrect 'constant' would still demonstrate the relative differences .

Rusty

 

[OnlyCummins]

George, I wasn't trying to be 'funny' (my attempts at online humor usually don't go over too well). Just trying to show you that break-in can take a good while. Obviously, the more & the harder the vehicle is driven, the faster it'll break-in.



Here's some info from Cummins' website:

How many miles does the Cummins engine require for break-in?



In general, 5000 miles. This depends on the type of driving the truck is subject to; hauling or towing a load will shorten the break-in period. Light duty hauling or "babying" the engine can delay full engine break-in for up-to 20,000 miles.

http://www.cummins.com/na/pages/en/products/dodgeram/faq/answers. cfm?uuid=44F01352-FC52-11D3-984A0004AC33EA57





You might find the following link useful:

http://dodgeram.org/tech/specs/axle/5sp_vs_6sp.htm

It's a comparison of the NV4500 & NV5600 but also shows recommended operating ranges.

 

[OnlyCummins]

Originally posted by Hohn

O. C. -- good post, lots of great info. Not sure what the snip means. .

Sorry ... I have a bad habit of assuming too much. I use "... snip... " to indicate that I've snipped (deleted) a portion of a response. Snip as in cut.

The dieselpage examples posted are misleading, whether gas or diesel. They would lead one to believe that you could get 30 mpg out of our trucks by simply swapping in a 3. 08 axle. ...

I respectfully disagree as I don't see how the inference can be made that 30mpg would be possible by simpling swapping to 3. 08's. Nowhere in my original thedieselpage quote do I see anything to support that conclusion. Here's the quote again:

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)

Peak torque for the 6. 5L TD is at 1700 rpm up to '95 & 1800 rpm thereafter (acording to Chilton's).

Peak torque for the 5. 9L CTD is at 1600 rpm up to '96 (that's the last year my Chilton's has).

... My point is simply this: taller axle ratios are not a quick ticket to mpg. Furthermore, this is true because of the diesel's varying fueling in response to load. A comparable gasser truck WOULD see higher mileage from taller gears, at least far more so than a diesel would.

Agree up to a point. The point that peak torque is significantly exceeded . A diesel will benefit from a reduction in rpm if that reduction returns operating rpms to a level at or just above peak torque levels. Can't speak to the relative benefits of reduced rpm for a diesel vs gas engine but improvements in mpg as a result of reductions in rpm are pretty much an accepted 'fact' for both types of engines.



Here are some examples:

I knew of someone with a pre-'92 GM C1500 Suburban that purportedly could get in the mid-20's in fuel mileage. And this from a NA (Normally Aspirated -- in other words no turbo) 6. 2L diesel with only ~145HP/~255TQ!!! But I think it had 3. 08's as the owner said it was a 'dawg'. Yeah ... I know ... too anecdotal but thought I'd offer it anyway.



http://www.landlinemag.com/Archives/2000/may2000/Swapping_Gears.html

... And since the engine makers and experts tell us we get better fuel mileage at lower engine rpm, ...

http://www.levelten.com/store/chry/items/converter/cr356_3000D_Billet_converter.htm

... The Cummins makes pike torque at 1600-1800 rpm. This torque converter allows you to say in peak torque range giving you the best performance & fuel mileage. We've seen 2-mpg increase, 3 second increasing in passing. Needed when in hilly areas & 5 wheel towing.

http://www.btinternet.com/~d.brooks1/specs.htm

... you normally get maximum economy from a car when its in it's highest gear at its peak torque.

http://www.motorhomemagazine.com/cf...n/thread/tid/897814/gotomsg/898290.cfm#898290

... Cruise at peak torque for max mpg, ...

And an additional quote from the thedieselpage:

The "GM Product Service Training Manual for the 6. 2L Diesel Engine" (#16015. 05-1D) has this to say about GM diesel fuel mileage:



"The diesel, like any engine, is affected by driving habits. Speed is more critical on a diesel than a gas engine. On the highway, in the 50-75 mph range, the fuel economy will go down about 3 mpg for each 10 mph increase in speed. A gasoline engine will lose about 1-1/2 mpg for each 10 mph increase in speed. This condition is perhaps the most significant factor in obtaining good fuel ecomomy. Fuel economy may vary as much as 5 mpg in a given vehicle with different drivers. "



Diesel engines have about half the useable rpm range as compared to a gas engine. A gasoline engine will produce acceptable fuel economy over a fairly wide rpm range. The diesel engine has a relatively narrow rpm range that will produce acceptable fuel economy.



The relationship of gearing to fuel mileage has to be the single most misunderstood aspect of the GM diesel engine. A typical 6. 5TD with 3. 08 or 3. 42 gearing will get in excess of 20 mpg at 65 mph because the engine rpm at that speed is right at the engine's torque peak of about 1700-1800 rpm. A truck with 4. 10 gearing is running at about 2250 rpm at the same speed and typically gets 15 mpg. A 500 rpm difference might not sound like much, but in a diesel, that's about half of the useable rpm range.

I am not familiar w/ the GM diesels (other than their tendency to explode when installed in late-70s cars), so I am limiting my comments to the CTD.

They don't explode any more but the '94-'00 do have above avg failure rates for the FIP, plus they tend to overheat, plus they tend to crack cyl #8, plus they almost universally have cracked heads, plus ... I think you get the picture.

Read above on George, who installed a GV overdrive w/ 3. 54s. no magic fuel mileage here from taller ratios.

There's an obvious reason why the GV OD has little or no impact on fuel mileage. Speeds are simply not high enuf to move cruising rpm that far from peak torque rpm. Assuming 245/75R16's (~29" overall actual diameter) tires the given ODR of 0. 73 & RGR of 3. 54 & a ~70mph cruising speed then that yields ~2095rpm which is not too far from peak torque. Picture completely changes when speed is ~80mph->~2395rpm which is far enuf off of peak to make rpm reduction worthwhile.

The "magic rpm" band of these engines is not so narrow, but rather from peak torque to peak hp. In stock form, that 1400-2700 rpm.

While it's true that the operating range is quite broad for a diesel it doesn't follow that fuel mileage is unaffected as long as rpms remain within that band.

Most gasser cars will max out a little fuel dummy on the downhill. But so would a diesel so equipped, and if you COULD get actual fuel consumption data at that point, you would see the diesel using much less fuel on the downhill. Again, diesels can run MUCH leaner than gassers.

I'm not that familiar with engine operation but I think that may be a moot point bc AFAIK there's essentially no fuel delivery occurring in the gasser going downhill so fuel mileage is basically infinity. Assumes accelerator isn't depressed & therefore engine braking is occurring (it was long interstate downhill coming out of west WY).

So, to minimize drag and operate within the engine's power band, optimum speed for my truck (3. 54, 265/75, dee) is:

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

SPD = (1400*31")/ (336 x . 73 x 3. 54)

SPD = (43400)/ (868. 2912)

Spd = 49. 98 mph.

You might want take a look at the following link: http://dodgeram.org/tech/specs/axle/5sp_vs_6sp.htm

& the recommended operating ranges. The table in the link suggests that 1400rpm might be considered borderline operation.



FWIW, in your calculations above, if the tires are actually 31" in diameter (would imply ~255/85R16's or equivalent) AND no recalibration was done for the larger tires then a correction factor would have to be used to determine what indicated speed (what speedometer shows) would be equivalent to the actual mph of 49. 98 you arrived at. Assuming a stock tire/rim combo is ~29 in diameter then the %age difference btwn the two would yield the correction factor. So 2"/29"=~0. 069, adding 1 produces ~1. 069, your correction factor. 1. 069 x 49. 98 actual mph = ~53. 4 indicated mph.



Am I windy or what!?

 

[geusterman]

I appreciate the effort for accuracy, OnlyCummins. The 6. 5 is definitely affected by rpm, as is our Cummins. On gassers there is an issue with vacuum that can affect mileage. Don't know that it applies to diesels other than through load (boost) and rpm (fuel). Bill Heath still has an 94 1500 6. 5 single cab that they moved the FSD on way back in 94 to a heat sink under the radiator. It will hit 27 mpg over Snoqualmie and back at freeway speeds. Geared high. No reason my Cummins should not be doing better than 14 mpg around 1500ish rpm. Guages will perhaps help but I think I need analysis. Have to pull that EZ and take in. Wonder if Cummins will dink me if I leave the EZ on?

George

 

[JRodgers]

George,



Just to add my . 02 on this, I have an 02 4x4 3. 54 also. It has the ETH and NV5600 also. I have the newer front axle without the center disconnect which keeps the front end turning all the time.



I have had the truck for about 7 months now and have NEVER seen any mileage over 19 mpg. My average is about 17 with city/hwy driving. My best are generally on highway trips when I let her run at about 2000-2200 rpm. This is a comfortable cruising speed of 70-80 mph.



I have experimented with trips where I cruise at lower speed with the engine turning 1700-2000 and my mileage was not better, in fact a little worse.



After talking to some folks that know these engines well, the general consensus was that they like to turn at about 2000 for best efficiency. Now depending on your particular engine, you may see a little better MPG at 1800-2400 depending.



My point is that 1300-1500 seems a bit slow. You should probably disengage the Gearvendors for a while and see what happens with your mileage.



Do you have a possible fuel leak somewhere that is screwing up your MPG calculation? I did at one time, and fixed it. Noticed an immediate gain of 1. 2 mpg average.



Definitely should have the MAP sensor and IAT sensor double checked too, becuase they could cause the computer to do strange things too.



Good luck.

Joe

 

[Hohn]

Great post, OC!!

This is the kind of thing that makes me renew my membership every year. You present a solid case.

I have satisfied myself that lower rpm SHOULD lead to improved fuel economy, even on a diesel. How did I do this? Well, pretty unscientific, but I would rev up to 2K rpm in 4th and hold it there. Without moving the accelerator, I would shift into 5th. Guess, what? It went faster. If my theory were correct, it should have maintained the same speed, just at lower rpm.

This tells me that the level of fueling required to sustain 2K rpm (probably even in neutral) produces more power than you need to maintain speed. Since you are producing more power than you need, you are wasting fuel to cruise at higher rpm in lower gear.



FWIW, I have a July-build 2002 (very late) ETH/DEE 4x4. I am pretty sure I don't have the axle disconnect. With only 13K miles on it (the last 4K with all synthetics) I can clear 20 mpg pretty easily. I seem to get 16 mpg around town (+/- . 5) no matter what I do. In addition to the mods in my sig, I have also installed the Fitch Fuel Catalyst, which I am doing a longer-term test on before I post. It has thus far SEEMED to have given me about 1 or 2 mpg more, and reduced smoke to boot. Like I said, I can still get almost 16 in town, even driving like Mario Andretti with the DD2s making lots of boost. I will post in depth on the FFC once I hit 30K-50K miles or so.



[/B][/QUOTE]

I'm not that familiar with engine operation but I think that may be a moot point bc AFAIK there's essentially no fuel delivery occurring in the gasser going downhill so fuel mileage is basically infinity. Assumes accelerator isn't depressed & therefore engine braking is occurring (it was long interstate downhill coming out of west WY).

This is something that I DO want to disagree with. Gassers DO burn fuel going downhill. A gas engine needs the a/f ratio to stay between 12. 7:1 and 14:1 or so. A gasser will use LESS fuel going downhill in spite of the near-constant a/f ratio, because it is ingesting less air mass. Because it's a throttled engine, the mix of air and fuel it sucks in varies greatly in density depending on the throttle position.

So the diesel burns less fuel downhill than it would uphill because it cuts the fuel rate back while air flow remains pretty much the same. A gasser cuts the AIR flow back downhill while the air/fuel ratio remains pretty much the same. The end result is similar behavior for exactly opposite reasons.

But there IS a difference. I don't know if ALL diesels operate this way, but on OUR diesels, there is no fuel being injected when your foot is off the pedal and engine RPM is above idle. It's almost like shutting down the engine. Fuel injection is restored so smoothly when you get back on the pedal that it's hard to detect-- but you can tell it's happening.

Under hard engine braking in a gasser, air is FORCED past the throttle blades. This is because the vaccuum that the engine can pull is limited. Once the air is forced past, the fuel is injected in a ratio proportionate to air volume. The gasser can't run infinitely lean-- EGT will skyrocket, detonation and preignition will set in, and exhaust valves get burned.

So, yes a diesel will burn less fuel than a gasser going downhill. At least, OUR trucks will!

It is interesting that cylinder deactivation systems are finally starting to catch on for gas engines. Computerized engine management systems allow the engine to shut down the spark and fuel to individual cylinders in a way that doesn't make the engine run roughly. Once these are on the market, THEN a gasser will be able to go downhill and use no fuel to do it!



HOHN

 

[RustyJC]

Well, actually, many gassers don't burn fuel going downhill if their RPM is high enough and the throttle is completely closed. That's because many electronically fuel injected gassers shut off the fueling during deceleration at 0% throttle position, only starting to fuel again around 1500 RPM (some even lower!) to prevent stalling.



Rusty

 

[OnlyCummins]

George, the example I cited earlier of the pre-'92 Suburban getting in the mid-20's on the hiway was one of the main reasons I bought a used '95 Chevy K3500 6. 5L TD HO (High Output) truck. Boy ... was I sorely disappointed as far as mpg! That's why I'm currently only "Considering ... " buying a '92-'93 1st gen. Mileage claims are so all over the map that I simply don't want to be 'burnt' again. My current inclination is to maximize the fuel mileage on a gasser. Both of my brother's get ~18mpg hiway with their '95 GMC K1500 Z71 Stepside & '96 Chevy K1500 Tahoe trucks both with the 350, autos & 3. 73's so I don't see why I couldn't achieve an equivalent or better fuel mileage -- even with a 4. 10 geared vehicle (that's why I bought a used GV's unit). Yeah ... I know ... towing mileage will suck but for me that's essentially a non-issue as I'll rarely be towing.



I wouldn't risk leaving the EZ on.



Hopefully it'll turn out to be something simple.



Good luck.

 

[geusterman]

JRRogers, you have encouraged me seeing we have similar trucks that there is some solution here. I need to restate that with or without the Gear Vendor, mileage is the same. Your comments:



"Do you have a possible fuel leak somewhere that is screwing up your MPG calculation? I did at one time, and fixed it. Noticed an immediate gain of 1. 2 mpg average.



Definitely should have the MAP sensor and IAT sensor double checked too, becuase they could cause the computer to do strange things too. "



are well taken and I will pursue them.



OnlyCummins, my gas Z71 got 18 at 55 mph but 14 at my normal 75. My hope is for my Cummins to ultimately beat that and the mileage of my 6. 5 with 4:10's 00 and 2600 rpm (14 mpg).

 

[OnlyCummins]

George, FWIW as I've already mentioned 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's mpg to low 16's mpg (corrected for the 235/85's) traveling at 80mph (indicated, actual was ~83. ?) with rpms at ~2800. Did so returning from WVA where I originally bought it & on a roundtrip to Canton, OH. I don't know what to say ... you're either having terrible luck with vehicles or drive like a maniac -- like me -- only I got better fuel mileage.