Okay , we bought a 2004 GMC Duramax. Truck is great, but I have a question. We finally got the 5th wheel that we are towing and took it out for a shake down cruise. We get a little over 12 towing -- not too bad. But our other general mileage averages only about 15. It seems to me we should be getting better than that when we aren't towing anything around.

We have been using Shell Diesel (for what that's worth). Are any of the diesel additives worth the money or is that just a waste of money? Any suggestions would be appreciated.

Portia

I just came back from St Louis to Nashville, and got 21.5 MPG (4 passengers plus luggage) with the cruise set at 65.

When I drive my truck hard, I get 16 to 17 MPG. Towing my boat, I get 11 to 12.

There are many posts where topic starters asked for MPGs under various conditions to see where they stand in comparison to others. That said, guys like me will always answer because we're slightly obsessed by our MPG figures.

I have tracked mine for about 30k miles (since I bought it used in Oct 04). Some city, some all highway, some towing a big 5th wheel. Here's my summary:
City-only driving, winter: 15.2-16.1...16.5-18.1 summer
Split City/Hwy, winter: 16-17.5...17.8-18.5 summer
All hwy, winter: 18.5-19.3.......19.5-21.0 summer
Towing 10k# 5th wheel: 11.5-12.5 hwy, fall/winter conditions.

I fill my tank TOTALLY full almost every time, waiting patiently while the foam goes down. I do not use any additives whatsoever, and my truck is 100% stock. I drive about 75 unloaded on the hwy and about 70 with the trailer. I've only towed about 1000 miles so my mileage info is limited.

Sounds to me like your 15mpgs is a bit low unless you drive it hard in the city. I am routinely getting 17.5mpg this time of year, city only.

"It's not an obsession, it's my truck.":muahaha:

Rip

Yeah, I thought it was a bit low too!!! Any ideas??? We don't drive it hard in the city. In fact we don't live in a city :). Highway we use cruise control and my husband drives really steady. Not familiar enough with a diesel to really pinpoint what the problem might be. Supposedly the truck was all checked over mechanically and routine maintenance performed. This was only a couple of months ago. The truck has (I think) about 18000 miles on it now.

Portia

Check the obvious things... tire pressure, air filter, fuel filter. I run my tires at 55psi unloaded, and 65psi when loaded.

I got 18.7mpg today. Best ever! Normally, 16-17mpg total.

I get worse mileage in the summer, due to massive influx of tourists that drive 1/2 of the speed limit.

If you have the Allison, you have to let it "learn" how you drive. I just bought a 2002 with 79000 miles and only got 15 to 16 in mixed driving, the next tank i got 17 to 18 in mixed driving. Give it a couple tanks, figure out how you want to drive it, and let the Allison figure out how you are driving it, then recheck your mileage and see what you have.

CYoung,

What size tires are you running, 245's?

Sarge

DuraMaxie,
The very biggest influence on mileage that I have seen with my two duallys, one gas and now my dmax is tread pattern on the tires. A rib pattern easily gave 1.5 mpg better (empty) than a blocky pattern on my 89 big block one ton. This was verified over 100k miles and several sets of tires. Haven't worn out the tires on the dmax yet but the experiment will continue.

Boy if you want 10 million responses just ask about MPG.....I have checked every tank since purchased and now at 35k. I drive 60/40% town / freeway and get typically 14-16. I have gotten at best 21-22 all freeway no towing. When I tow my lightweight trailer all freeway, I typically get 16-18....my normal freeway speed is 65-75...for what its worth

Duramaxie,

GM made a mid year engine change during the 2004 model year run. The original Duramax engine is referenced as a "LB7" and the newest version is known as a "LLY". These are the GM order codes. This came about due to emission law changes effective January 1, 2004. Look at your truck's VIN number. If the 8th digit, starting from the left, is a "1", then you have a "LB7". If the digit is a "2", then you have a "LLY".

You need to know this when comparing milage to others posting on this board. It appears a LB7 will get about 2 or 3 more highway miles per gallon than a LLY, unloaded. The difference appears to drop somewhat when loaded.

I would suggest you add a "Signature" containing the deals on your truck. This will allow the experts on this site to give you a much better answer. Is your truck a dually, is it a crew cab, does it have the Allison, does it have a heavy tool box, are you running standard tires, is it lifted, etc....... These all effect fuel milage, most of them for the worst.

Assuming a standard unmodified truck (Ext. Cab, single axle, factory size tires inflated around 60lbs, no load, Allison), running empty, on flat ground, highway only, around 65 MPH, I would expect about 19.5 mpg on an LLY and about 21 or 22 on a LB7.

You need to be careful when evaluating the postings concerning fuel milage. Many do not list enough details to reach a conclusion. A good note should contain details on the truck, state if the miles were hand calculated (the DIC display is not always correct), how the truck was driven (speed, highway, etc), and if the tank was always filled to the top. After reading a few hundred of the fuel milage posts, you start to get a feel for it!

Here are a few things to consider in regard to improving fuel milage. a) These trucks are very heavy vehicles and it takes a lot of fuel to get them moving. Try to hold a steady speed. b) Our trucks are as aerodynamic as a brick on wheels. The faster you go, the faster the milage falls. Seems the most efficient cruise speed is around 50 mph. Milage starts to fall rapidly over 65 mph. c) Check your tire inflation. An empty truck, standard tires should have around 55/60 lbs of air. d) Maintain the truck -- fuel filter, air filter, trans filter, fluids, alignment, etc. e) A power chip of some kind may potentially add 1/2 to 1 mpg. f) A fuel additive with a cetane booster may help slightly. g) Summer diesel (#2) will be 1+ mpg better than winter diesel (#1).

BH hit the issue correctly.

If you have the LLY you are doing very well. If it's the LB7, it could be better. I have friends with the LLY and I get much better mileage with my LB7

I have 04 lb7 crew cab, for what it is worth 19 arown town- 21 highway- 12.5 towing 9000lbs way to fast. Heading to california from new jersey next week, I swear I'm going to stay below 70 towing! Truck pulls and handles so good it is very easy to go to fast. My .02

Depends mostly on gear ratio.I get 20/14 with 3.73 gears, my brother gets 15/13 with 4.11

21 mpg last weekend on 1500 mile trip,NC to NY and back, 5 passengers and luggage, all highway, 55-70 mph. Stock (edge removed) with fresh oil and all filters. Fuel price I paid ranged from $1.97 (VA) to $2.37 (PA).

Yes Sarge, they are the factory 245/16's that came from the factory.

Portia

Best I've gotten in the past 8-10 months is a crappy 14.5. But all I do is alot of short trips with maybe some longer 50-60 mile trips. I had gotten 19 when I first got it last July. Stealer says they don't know what could have caused the drop. I just deal with it now.

Something I've often wondered about concerning fuel mileage on a diesel is the use of the cruise control. Back in the 70s a car mag (IIRC-Car&Driver}did a test on gas powered cars.
The test was for hwy mileage using several different techniques of driving to maintain speed. One method was not using the cruise control, but trying to maintain speed with the throttle. Second method was using cruise control normally. Third method was a steady throttle with enogh throttle applied to maintain the desired speed on the uphill side and holding this same throttle on the downhill side regardless of speed attained (within reason and traffic permitting, of course).
IIRC, the result was better mileage with the 3rd method and it was commented that this was the technique used by most long haul truckers, and that they were upset about the then national speed limit of 55mph and that it was costring them extra fuel when they had to back off on the downhill side to avoid a ticket.
It would be interesting to check this out today with our trucks. Unfortunately the cost of fuel has curtailled my driving to what is absolutely required and that ain't too much in over thje road.

On my Dodge (CTD, auto, 3.54, 2WD, 254,000 miles) Cruise makes a HUGE difference. Usually 2 to 5 :eek: MPG better with the cruise off. I havn't had the Chevy long enough to do any kind of real test.

Does winter fuel really get worse milage than summer? I filled up in Happy Jack a few weeks ago got 2mpg better than usual using their winter that was still in the pumps. Unless I misunderstood what winter fuel was. (which is very possible)

Winter fuel from what I understand is better than summer fuel which #2 Diesel and winter fuel is #1 which is suppost to aid in resisting gelling (waxing of the filter, lines and fuel tank) in cold weather. I know when I worked for a Peterbilt (and yes that is the correct way to spell Peterbilt not Peterbuilt!) when they would bring the new trucks from texas they would sometimes not run in the winter time till we pulled them in the shop to warm up cause in texas they don't have to worry about gelling cause it never gets very cold down there so it still had #2 Diesel fuel in it and it was all gelled up. The reason I think your are seeing less MPG in the winter time is cause most and I hope all of us let our trucks warm up before we start to beat the crap out of them and all diesels just run better and like the warmer weather better than the cold and I think that you will see the same drop in fuel mileage in gas engines ( not motors, motors are not internal combustion....only ENGINES ) in the winter time. But the winter fuel is suppost to be better. Just my .02

Real men have sticks that go all the way down to the floor!!http://www.dieselplace.com/forum/images/smilies/driver.gif

Sarge,

Running stock size BFG AT's

Another thing you need to consider is how many miles do you have on it. I have a friend that said his did not break in until 50 or 60K miles. He said then it improved with almost every fill and he now gets 22 to 24 mpg. Keep that in mind.

Thats true too.

I have had my 03 for about 2 weeks now, and bought it on my wedding day and we drove it on our honeymoon. On the way home, all highway miles, through the mountains and driving from 50-80 mph I averaged 18. This last tank of driving around in town mainly with may a third of the miles being highway I got around 15. I keep checking each tank, trying to let the Allison learn my driving habits.

I get 16.5 in town with out the heavy foot and 19 with the cruse set at 85

I'm pretty disappointed in the mileage so far. I'm running 285's so the actual mileage is probably a few percent better than what the DIC tells me. That said, I'm getting about 7.5 towing a heavy toy hauler at about 65-70 and am lucky to get 13-14 unloaded. Truck runs great, tows great, stays cool, etc. . . No obvious problems in that department, it just sucks down the #2.
Any ideas?



Gar

biggar, your mpg numbers are similr to mine. I've had an 02 and an 03 (both 3500 4x4 crewcabs). At 65 mph I may get 17(w a/c off), but at 70 or above I'm at 13 to 14 unloaded. In town is around 12.

Memorial day weekend I pulled a trailer with a swamp buggy on it. Easily weighed 14,000 lbs. Staying at 60 to 65 mph got me 8 mpg.

Obviously anything I can do to improve the milage would help. I'm soft-footed and baby the thing trying to get the best milage possible.

Mine's an LLY, so it probably doesn't mean too much on this forum, I just searched mpg and then replied to a few threads. I think my mileage is still better than it would be driving a gasser with similar power. Unless they build a 4 wheel steering version with the duramax, I'll be keeping this truck a long time.
I really like the way it works, mileage be damned.



Gar

It Sounds To Me Like Biggar Has An Lly Also. With The Finger Plate And Stick. Dont Think You Need One On A Lb7. For What Its Worth. My 02 With The Lb7 Got 13-14 Mpg Towing 7000lbs. My 04 Lly Gets 9.5 Towing The Same. Its Now For Sale. Picked Up An 03 Lb7 Over The Weekend. drove it home last night. got 22mpg with 285 tires add 6% for tire size that makes it 23.32 mpg.:cool2: thats all highway about 60 miles. hope it does good towing :eek:

Its wierd my truck gets about 17 around town, and 24-25 on the highway. It does well on mileage.

If I drive 65 steadly, I get above 22 mpg. But when I drive hard the fuel mileage drops down to 18 -19.

If I drive less than 70 it seems to stay above 20mpg.I haven't had a good long trip that I've kept it that low on for a while.Running 75-85 it gets around 17-18 and running 70-80 towing 8k it get's 12-14 and towing 15k+ at 55-65 it get's 12-15. My Dad's gets 19 at 75 and 20-22 at 65 and under and similar to mine when towing.I have a 6spd he has an Allison. I could get better but I drive mine pretty hard and he is very carefull with his.

21 MPG doing a 35 mile trip to work each day including about 4 miles of non-highway, the rest at 65-70 MPH. 15 MPG at 65MPH towing a Ranger Boat weighing about 4500lbs

Biggest thing I've noticed is taking it over 75mph knocks a few miles of a gallon. 73 or under does great mileage

I must have a good one. Been getting 20.1 mpg lifetime average. Have an '03 with a 6spd manual, 19,000 miles, stock everything, using Rotella T Synthetic. I too fill the tank "all the way" every time to make sure that I don't skew the numbers. My high is 23.4 and I had one short tank of 13.7 (winter time and didn't shut it off for about a week). I've had 4 or 5 tanks above 23 and usually get 21 to 23mpg on the highway. My 4 mile one-way commute to work is getting me between 17 and 18mpg. If towing, it drops a little, but not too much. Been pulling for an RV place, and most of the trailers they put me with are 8k to 10k bumper pull. Usually get 18mpg at 65 - 70mph with trailer in tow. I've heard that the ZF manuals get better mileage after they've broken in. I can't wait...

Toybreaker

Here's the info for comparison...

'03 2500 HD extended cab, LB7, 4x4, stock upgrade tires (245 75 R 16 Firestone Steeltex), 6 spd ZF, VDO pyro and boost gauges, tool box (in bed), rubber floor, crank windows, open bed, drive with tailgate up, manual mileage calculation, no fuel additives. Oh, and it's white... don't know if that matters :) Been keeping track of gal/hr too...

I average 20 miles per (imperial) gallon with my rig, approximately 60% highway and 40% city driving. During winter months it drops down to 17 MPG. Towing my 5th wheel RV trailer weighing in at 7,800 lbs MPG is 11 to 13 depending on wind conditions.

2004 Silverado Ext Cab Duramax 3.73, 5 spd allison, no mods

Have any of you with varying mileages, also broken in your engines differently? Was just curious if there would be a difference in mileage for those that break in easily, versus those that just get in and put it to the floor right off the dealer lot?

I drove mine during break-in about the same as I do now. Didn't beat on it, but no babying either. I did hit redline once or twice during the first couple thousand miles. Did the first oil change at 1,500 miles just in case there was any nasty stuff in there.

Does anyone know when the official break-in period ends?

'03 2500 HD extended cab, LB7, 4x4, 6 spd ZF

One thing I have noticed is the mileage reported via the DIC is always a few MPG better than hand-calculated mileage.

Lifetime average as reported by the DIC is 17.7 MPG
Lifetime average for calculated mileage is 15.9 MPG
Lifetme = ~28,500 miles.

I also have noticed that the difference is greater with the Juice/Attitude in place.

With my Hypertech, hand calculated 18.7 mpg. Pretty good for my circumstances, I think.

How does the 4 wheel drive affect the fuel milesage. I just bought a brand new 2005 Chevy with the Duramax engine, 4 door and long wheelbase. The sales guys said it does not affect it much with today's technology, that true?

Paul
Texas

How does the 4 wheel drive affect the fuel milesage. I just bought a brand new 2005 Chevy with the Duramax engine, 4 door and long wheelbase. The sales guys said it does not affect it much with today's technology, that true?

Paul
Texas

The extra wieght of transfer case and front differential will certainly affect mpg. Not hugely but it will affect it. I got back from a 250 mile trip towing 6500lb 5th wheel with my truck. Just did manual calc today , 14.2 mpg. Truck has 143,000 miles. I do use stanadyne treatment. This is double the milage I got with my old gas truck so I'm very happy with this. Yes its 4x4.

Paul,

Thanks for the reply, that's pretty good mileage pulling a trailer! My new truck has the new version LLY engine, I just noticed I asked my question in the older version thread! Sorry.

I wonder why the board doubled my last post, I went into the advanced area and turned on the email notification. Could that have done that?

Paul

My mpg's are 16-17 around town and 7.5 -8.0 towing my 11000lb 5th wheel camper. I think the towing mgp's stink. I was hoping for at least 11.0 towing.