Fuel Economy

[George Gozelski]

This isn't really a problem but I was wondering what you 3500 D/A CC 4x4's are getting for fuel economy. My old 6.5 CC (non dually)used to get 16 mpg! I'm wondering if it's the extra weight or what? My buddy with the identical truck to mine is equipped with the 6 speed and gets 18mpg to my 14. If I did the 4" exhaust thing would that help get it up there? tongue.gif

[Swayse]

My fuel mileage really sucks. 350 mile trip at about 85mph and I got 14.4mpg and I've got a 4" exhaust. The best I ever got was 16.7mpg and that was with the stock tires and exhaust driving very nice and going 70 mph. With 285/75/16 on my 2001 Dodge Cummins 5 speed 4x4 driving 75 to 80 mph I could get 20 to 21mpg on a trip, and that was with injectors and a box and a 4" exhaust. Maybe I need another Cummins.

[markf]

George, I am right there @ 14 mpg with my 02 3500 CC D/A 4x4 GMC. My wife drives a 98 one ton 5-speed cummins 24 valve and gets 16 so I don't feel too bad. Most of the 3500 owners I have talked to fall right around 14-15 mpg. My truck is all stock to date as well.

[bryantch]

Swayze,

Try putting the factory tires back on (if you still have them) and you will most likely gain a couple of miles per gallon. I replaced my 265 Wrangler MTS with the factory tire/rim package due to warranty issues and immediately gained 2 miles per gallon. my truck is currently running completely stock as far as tires/engine mods are concerned with the exception of being straight piped. I keep extremely accurate records of my mpg as it is one of my company vehicles and I average 14 mpg and get 17 on the highway as long as I stay below 75 mph. Peak torque on these motors is at 1800 rpms which is always the point (peak torque) at which you will get the best fuel economy. Unfortunately the fuel economy of diesel engines (more so than gas engines) will drastically decrease the more you increase the engine rpms over the peak torque number. On my dmax 60mph+/- is 1800 rpms.

Unfortunately putting larger tires on the dmax does not help with fuel economy at all. The allison just does not like them...and wont until gm releases the codes to account for different tire sizes. My ford on the other hand has benefited from 295 tires by essentially lowering the final drive ratio...this should work with gm but the general hasnt seen fit to allow it.

FWIW, my .02, my experience, hope it helps...

ian

[Bobcat698]

I have been getting 19MPG so far empty & about 17.5-18MPG pulling combined with 1/2 pulling a 8,000lb trailer & 1/2 empty.

I have not run a full tank through it yet though in this chilly weather...

Garrett

[Runaway]

Bobcat, your 2500 should get better mileage than the 3500s, You've got 2 less tires of drag. :;

[56Nomad]

Hi all,

George asks about his 3500 4x4..... not a 2500
Every time there is a posting about mileage it seems that
we start comparing apples and oranges.

Hi George,

With my 3500 4x4 around town I get about 15 mpg
With my Lance camper (10ft) I average about 13 mpg
Unloaded and on the freeway I average about 17 mpg

From my recollection..... the guys who have gone with the
4" exhaust have not reported any mileage gain.

[Bill McGouirk]

My fuel mileage really sucks to. I have a 2001 4x4 std cab with 285-75-16 BFG MT I love the look but I only get about 15 mpg. It's still a lot better then my old 6.0 chevy it got 11 or so. It did get better after I put in Delvac 1. Oh well it is a lot of fun to drive.
Bill

[Swayse]

Oh heck yea it's fun to drive, feels like a sports car. But I am not putting my small tires back on, truck looks way too small. Wife makes fun of me then, her truck feels bigger than mine now, and hers is a half ton(2002 1500 Dodge 4x4 with the 20s).

[SaltH2OFisherman]

In reading the post above, I was wondering if those who have changed to bigger tires adjusted their milage? If not you will get less mpg with bigger tires.

[trout49]

I have 7000 miles now and my mileage just keeps getting worse. The first few tanks I averaged around 17mpg. Now I can barely break 14mpg, if I keep it under 55mph. If I dare try 60 it drops to 12mpg. Still runs good. Still like the truck. I'm just waiting for 15k on the odo. then see what I get. I here it takes that long to break in.
I even tried a new air filter, amsoil of course. Like I said it drives good, pulls good, just sucks on mileage. Dealer says all is well. Never any codes or problems.

[Paintdude]

I have always wondered if a oil analysis would help tell a guy getting poor fuel mileage something the dealer cant seem to find..

Just a thought..

[bryantch]

With regards to tire size changes and the math that needs to be done to correctly figure mileage...in short, going to a 265/75 tire (in my case) means a 2.2% decrease in revolutions per mile (683 revpm factory vs. 668 revpm with the 265's). This small change in tire size will NOT appreciably change the mpg calculations. If your odometer showed 300 miles, you would have actually gone approx 306.6 miles. If you used 20 gallons of fuel the calc without factoring changed tire size would be 15mpg. the calc figuring the tire size difference would be 15.33mpg.

Going to a BFG (a popular tire upgrade - each manufacturer sizes their tires differently) 285/75 the revs per mile would be 632. This represents a 7.5% decrease in rotations per mile. Using the 300 mile odometer number again, you would actually have traveled 322.5 miles. Using 20 gal of fuel - not figuring tire size diff - 15 mpg. figuring 7.5% diff in tire size (again this only applies to the BFG AT KO 285/75) would be 16.13 mpg.

By contrast the Dunlop Rover AT 285/75 rev/mile is 658 and the Bridgestone Dueler AT is 633. Point is that the SPECIFIC TIRE you use will determine the calculation, not merely the tire size.

I dont mean to be "preachy" but I did figure the tire size into my mileage based upon the tires I put on the truck. In MY case putting the 265s on reduce my fuel economy 2 miles per gallon, the tire size correction calculation is not worth figuring...and they caused tranny related probs. Unfortunately the General only set the Allison up to work correctly with a tire that has approx 683 revs per mile (Stock 245 is 683 revpm and stock 215/85? is 680 revpm)and wont allow the service dept change the ECM to account for after-purchase tire changes (to make the truck work better and LOOK BETTER!).

My .02...

[George Gozelski]

Thanks for the replies! I thought all the "touting" of the Allison transmission from my local dealer was the end all of what to purchase; the 6 speed or the auto! He told me the economy gains from the 6 speed over the Allison wouldn't be noticeable! If I could get in my time machine and go back to last October, I would probably have bought the 6 speed! I like my D/A mind you but I feel it should be getting better mpg. Like some of you, I once owned a Dodge Cummins with the 5 speed manual and would routinely get 20-22 mpg! Will propane give me the added mpg gains and/or using synthetic oil?

[Onebigcanuck]

Read my reply in "Warning 2003 concerns " this could be part of the poor milage in 2003's

:rolleyes:

[FrozenAKJoe]

Thank God! Finally, a topic about mileage on the 3500 CC LB D/A! We always see posts about the fantastic mileage of the 2500, but we all know that the 3500 costs us miles per gallon.

I don't feel quite so bad now. I thought I had something wrong with my truck since I wasn't getting the huge numbers that most are talking about. I got 17 one time this last summer, but I think that was a fluke or maybe the nozzle shut off too soon, because it only happened once.

My driving is pretty much 50/50 city/hwy. I feather the pedel and I never drive faster than 70 (usually 60). (I try to keep the RPM around 1800 as well, but sometimes the people want to run me off the road unless I drive faster.) I just added a tonneau cover to my bed, so the mileage is helped a little bit there (I noticed about a 1 mpg increase.)

Empty (summer): 16-17
Empty (winter w/ bra on): 12-14
Loaded w/ 11.5' 4000lb camper & 2800 lb trailer(summer): 12-13

I was hoping for something a little better, but compared with my '99 Dodge 3500 V-10, I'm not complaining! Dodge stats:

Empty (summer): 9.2
Empty (winter w/ no bra): 6.7
Loaded (same package but minus the trailer, in summer): 6.3

[Stage 1]

Just to add a little data under different driving conditions, (2 wheel drive, and new truck w/only 2600 miles) but it is towing 12K to 13K pounds in 30' enclosed trailer, and do not think the 2 vs 4 wheel is a big factor at this level of load. For two thousands miles of towing averaged per fillup from 9.2 to 11.2 MPG. The 9.2 MPG was in mountains running as hard as 2nd gear at 3,000 RPMs (lowest instant MPG was 3), and 11.2 was up and down hills.

Have very little experience yet with non towing fuel mileage. Preliminary thoughts are that stopping and starting really hurts fuel mileage. This is based on watching the instant fuel mileage readouts. Do not have any good data yet, but instant reading on level road wo/wind at 65 MPH shows between 18 and 22 MPG. My only changes are tires and air cleaner, as noted below.

[Swayse]

I use 7.2% when I figure milage. Love the motor, hate the tranny. I wish I would have got the 6 speed also.

[SaltH2OFisherman]

For the people who owned a dodge that claim 20-22 mpg in a five speed, I have a question. Was it a 4X4? I owned two one 2500 5 speed and one 3500 5 speed. Both 4X4. I would average 13-14 in the 2500 and 14-15 in the 3500. The Duramax K2500(yes I know there is a difference in the 2500 vs. 3500 and we are talking 3500s)is getting in the 17-18 range without calculation for the 265s. Same driving conditions are the same. All of this with the Allison.

[Swayse]

Yep, 4x4 2500 quad cab 5 speed with 285/75/16 on it. My GMC is a single cab 4x4 with 285/75/16 on it.

[Burner]

Hey Fellas,

For all of you that have the 03'fuel milage stuff. --- Do not trust it! --- :eek: I have checked mine at every tank and it has been wrong! On the first 8 tanks of fuel it was off an average of 10% or more! Just last week the blasted thing ran out of fuel! Before leaving work it said 108 miles to empty...20 miles later I RAN OUT OF FUEL :mad: !!!
:mad: If GM doesnot get their SH1oT to gether on some of these issues or at least the "new" version (04 truck), I'll get a Dogde or Ford!!! I'm tired of not having the ground clearance I need and not being able to do a dag-gum thing about it! :mad: This truck has what you would think is the goodies package. Well, it reads great and looks good on paper untill you actually have one! I have 265's and "Still" drag the ground! 45,000 dollars for a 4x4 truck that is not as capable off road as a Subaru wagon!!! :(

I'm really ticked at "The General" right now. Why I paid "more" for this truck than a Dodge or Ford....I just don't know. :(


Burner~~~~~~~

[fredw]

as for my 03 fuel milage i have seen a big change in the last 1000 km, i am now at 9500km on truck and run a 90 gallon slip tank that always feeds into the primary tank, was showing till lately about a max of 580 to 630 km on a full tank, now am showing 750 t0 800km(about 500 miles)per tank and seams to be getting even better, without any different changes to my driving, that works out to about 22.7 miles per cdn gallon, cannot complain about that, i feel that the large tires, k+n air filter and bully-dog give me this advantage, as for my driving i am always on cruise at 117 km or 73mph,on my garman gps, and am not pulling anything, i have had now three duramaxes and all millage have been about the same once broken in
thanks fred