View Full Version : Gas Milage
Onebigcanuck
07-27-2002, 15:47
Just returned from a 900 mile Trans Canada Highway trip through the Mountains.
I've all the updates to the computers and empty I averaged 29 MPG at 65 MPH. Got to love that Duromax/Allison
Ahem ~ We take that to be Imperial gallons? ~ odoh
13,500 kms. on the clock and have never done better than 21.5 miles per imperial gallon.
It would be nice to be able to look forward to that kind of mileage!
Have you heard of others getting in the mid to high 20's?
Onebigcanuck
07-30-2002, 11:30
You are correct:
Ahem ~ We take that to be Imperial gallons?
Truck has 32,000 kms.
I've got 32,000 kms on mine and my best so far is 25.5 mpg (imp) but that was at 70-75 mph on the 401 freeway for a distance of 600 kms. I expect that if I can keep it at 65 I would do better.
City and shortt highway close to town gets me 22 to 24 mpg.
:D :D :D
I would love for anyone that claims to get that kind of millage to pass through my neck of the woods. That way we can go for a ride and I can show you that the fuzzy logic math you are using is plain wrong. No way is any dmax going to get better than 18mpg and I say that loosly since the best I have ever seen is 16.
RWHP
--------------------------------------------
fuzzy logic math?
---------------------------------------------
2+2 always has and always will be 4--nothing fuzzy about it. THE ONLY QUESTIONABLE part is if the FUEL IS BEING FILLED TO THE SAME LEVEL EACH TIME?? If it is, then Gals divided into miles = correct MPG. It appears that I was wrong to think the 6 spd. manual would get better mileage than the Allison. But regardless, I once ACCURATELY checked my mileage at 65mph, all interstate and got better than 19 mpg. I am driving the "biggest of the big P/U"(see sig.), carrying a ATV, total weight at 8800+#'s. :D :D
Black Dog
07-30-2002, 13:34
I will guarantee you that my 2500HD regular cab 4X4 will easily get 21 mpg (US gallons) on the highway. Checked many times to average the refill level.
Ive had a 3500 D/A for over a year now with 27,000 on it & never passed the 18 m.p.g yet.Averaging around 17 m.p.g.Pulling I get around 9-12 m.p.g.,depends on the pulling conditions.It has averaged the 17 empty ever since I got it.The 2500 may get better fuel mileage cause they havent got the weight the duellys have.I could see 19 with the 2500s,but no more.Im talking strickly Diesel here & not gas engines with the Allis. Hammer. smile.gif smile.gif
White Knight
07-30-2002, 14:53
I have almost 14,000 miles on my D/A. My last two fill-ups gave the following results:
453 miles; 23.127 gal; 19.59 MPG; avg. speed 55.92 MPH
144 miles; 7.351 gal; 19.59 MPG; avg. speed 62.61 MPH.
Every time I fill up I dribble in the last couple of gallons of fuel until I can see the fuel (not just foam) in the filler neck. I am confident of the above figures. As you may assume from the average MPH, all but a few miles of both tanks were on the Interstate.
I have just about 10,000 on mine, I average about 18.4 around town, 22.3 on the highway from full tank to empty at speeds between 60-70.
It is possible! Driving a friend's Duramax on the highway (From Seattle to Montana) and seeing the mileage and power is why I bought mine!
Scott
[ 07-30-2002: Message edited by: SDWA ]</p>
hgiesbrecht
07-30-2002, 20:59
My Duramax/Allison has 6684 kilometers It is a 2001 4x4.Last tank of fuel averaged 25.92 Miles Per imperial. 468 kilometers=290.8 Miles. 50.99Lires=11.22 Imperial Gallons. The average equals 25.92 Miles Per Imperial Gallon.Most of the driving was Highway Speeds of about 100 Kilometers an hour which converts to approx. 60 to 65 miles per hour.There was some city driving but very little.
i have seen once 23 MPG US. that was driving 65 though.
Vantage #51
07-30-2002, 21:48
2002 D/A 3500 Crew , From Nova Scotia to central Virginia with aproximately 11,000 km on it. 3 tanks all at 19 to 19.6 MPG imperial, doesnt seem to have gotten any better now with 21,000 km on it either. Was really expecting more, as gasser 350 99 suburban that was replaced would do the same.
Just returned yesterday from a 2800 mile trip Santa Fe to California (northern) with a lot of Sierra mountain work. My truck was emty except for a full cab of two adults and two dogs and all the 'stuff'. It has the latest PCM and TCM updates as of the day before departure. I-40 speeds were 80-85mph (indicated) or approx. 2400 -2500rpm. Mountain speeds were obviously much less. Two lane, non-freeway was about 70-75mph. Best milage was 18.9, worst 16.1. Total trip milage average was 18.1
I only wish I could get numbers like that. I just bought a new '02 Crew SWB 4x4. My first trip, cross country interstate (I20/I10), mostly 70-80 mph, and I averaged around 15 mpg. :confused: I must say I'm disappointed.
Bill McGouirk
07-30-2002, 22:07
I just got back from a road trip and got 27.25 MPG on the freeway, oh that was in my wifes Honda. I agree with RWHP I check my MPG all the time and have never got better than 17 to 18 on the freeway. I have a 2001 std cab 4x4 and there is NO WAY a Demax can get better MPG than a 2002 Honda accord with a 4 banger under the hood NO WAY. FUZZY Math or BS You be the judge.
Just My 2 cents
Bill
imported_
07-31-2002, 00:14
12.25 US MPG towing 8500 lb step up fifth wheel across saskatchewan...and empty i consistently get 19.5-20.5 miles per us gallon.... and if y'all do not believe it..... c'mere and take mine for a week!
SoCalDMAX
07-31-2002, 00:52
Hi Guys,
I just want to point out 2 things here. The Dmax is very sensitive to speed (rpm) when it comes to mileage. I've seen as high as 20mpg at 60mph (3 tanks in a row) and as low as 15 mpg at 85-90mph. I usually avg. 16.5, and I don't think about mileage when I drive. How one accelerates makes a difference as well. Smooth always saves fuel.
The other issue here is our Canadian brothers aren't using fuzzy math. they're using Imperial gallons. To convert to US mileage numbers, multiply by .83267. If someone's driving a reg cab 2500 SB 6spd and driving smooth at 60mph on level ground with 80psi in the tires, that explains a lot.
I don't think anyone's pulling anybody else's leg about it. There are also wide variations in mileage for some unknown reason. The exact same variations and discussions occur among PSD and CTD owners. There's no concrete theory explaining it that I know of.
Regards, Steve
mackey_62
07-31-2002, 05:28
I'm not planning on being in LA area anytime soon, but fuzzy logic math tells me I'm getting 19 to 19.5 pretty consistently. Oh, lets see, miles traveled / gallons used = fuzzy mpg :D
And every time it's FULL!! Actually spill a little bit every time.
Hey, maybe all that polluted city air has something to do with your poor mileage. Poor truck needs a little o2. I can hear it coughing now :D
woundedbear
07-31-2002, 14:38
Felt I needed to post in order to validate some of the mileage claims that some are having a hard time believing.
I, too, was extremely interested in mileage numbers when I was contemplating my purchase (see sig) last November and didn't know who or what to believe. I would like to state that I have absolutely no reason or motivation to lie and there is nothing fuzzy about my math. I simply want to relay my truthful numbers to those who chose to believe.
Regarding fuel fill protocol, I also fill my tank each time so that fuel (not foam) is visible in the filler neck. Yes, it is time consuming, but my desire for accurate mileage numbers makes this a necessary step. Also, not only do I record per tank mileage numbers, but I also have my spreadsheet calculate 5 tank moving averages of my mileage, thus virtually eliminating the tank to tank variables.
Anyway, I have been extremely happy with my mileage thus far. Below is an excerpt of my spreadsheet for my last 8 UNLOADED fill ups:
TANK MPG / 5 TANK MPG AVERAGE / TANK MPH
19.364 / - / 40.390
19.238 / - / 36.759
19.405 / - / 34.266
18.239 / - / 35.171
19.264 / 19.102 / 38.823
19.172 / 19.064 / 37.374
19.144 / 19.045 / 32.667
19.804 / 19.124 / 39.478
As you can see by the TANK MPH numbers, these MPG numbers were obtained with highway/city driving. I feel achieving 20+ MPG US is an absolute reality driving with a TANK MPH of 55-65.
For the most part, my truck is stock. Things that may be helping my mileage numbers are the following: Amsoil 2 stage air cleaner, Amsoil 5-30 oil, soft tonneau cover and .25 ounces of 50% FPPF Total Power Deluxe / 50% FPPF 8+ Cetane Improver per gallon of diesel.
[ 07-31-2002: Message edited by: woundedbear ]</p>
I read an article a few months ago about Diesel fuel economy. I thought it was on this site but I can't find it. The crux of what it said was that for every 10 mph speed increase above 60 mph you can expect a decrease of 3 mpg in fuel econonmy. The reason give for this is that the diesel engine operating rpm range is very narrow, ie. 3100 rpm compared to a gas of say 5500 rpm max. With a 10 mph increase in speed the rpm increases a greater percentage of the range which means a greater increase in torque and horsepower which consumes more fuel.
The guys that are driving 70 - 80 mph are getting on average 3+ less mpg than someone driving 60 - 70. Driving habits account for a large difference in mpg because of the same explanation above. Driving with a heavy foot consumes a lot of fuel because the rpm is being pushed up.
When I drive sensibily on the highway I will get 20.5 + but if I start pushing it the mileage drops and the same thing occurs in city driving. This truck will get good mileage but you have to drive it right to get it. I averaged 10.25 mpg towing a 32 ft travel trailer (8600 lbs) over 3450 miles in desert, hills and mountains. It beat the h*** out the 454 Suburban (5.5 - 7 mpg)that I traded for the Duramax.
The F**d owners I have talked to wish they did so good. The D***E drivers don't brag much either. Be good to it and it will be good to you. Just remember it takes fuel to create power, more power the more fuel.
What a truck.... very difficult to believe! My 2001 HD DMAX/Allison is averaging only 16.76 - 24k on the truck and 1431.5 gallons of fuel used - kept all the receipts - kept track since it was new.
thanks Greg
WHAT IS SO HARD TO BELIEVE?!! Why can't some people simply understand that vehicles are like people. THEY ARE ALL DIFFERENT. Accordingly, some get better MPG than others. Heck, I bought the 6spd. for better mileage, but most of the Allisons get better than I do. It is just the luck of the draw as to whether you get one that is average, better, or worse on MPG! JUST BECAUSE MINE DOESN'T AVERAGE 20MPG DOESN'T MEAN THAT NOBODIES DOES!
"The crux of what it said was that for every 10 mph speed increase above 60 mph you can expect a decrease of 3 mpg in fuel econonmy. The reason give for this is that the diesel engine operating rpm range is very narrow, ie. 3100 rpm compared to a gas of say 5500 rpm max. With a 10 mph increase in speed the rpm increases a greater percentage of the range which means a greater increase in torque and horsepower which consumes more fuel."
I don't doubt that the article said that, but just thinking about it from a physics point of view, I have a hard time with it.
The amount of power it takes to propel the vehicle at a given speed follows a very specific formula. Horsepower required goes up with the cube of the speed increase, that's a standard formula you'll find in a physics textbook. So, for example, to go 70 instead of 60, you take (70/60) cubed = 1.59, or 59% more power. It's true of all vehicles, whether they're gas or diesel or electric or whatever. The basis of the formula is that wind resistance goes up with the square of the speed increase, and it takes another power factor just because covering more ground in the same amount of time requires more power even without changes in resistance, so you end up with a cube function.
Where I have a hard time with the powerband width/excess power explanation is that if the engine is creating more power than what it takes to support a speed, the vehicle will go faster. Likewise if it's not creating enough, it'll go slower. Fact is, a motor isn't necessarily making more power just because it's rpm is higher. You modulate how much power is being generated at any given rpm with the throttle position (remember a dyno sheet is generated with the throttle wide open). So really, I don't see how the width of the powerband has anything to do with it. The horsepower is controlled by your right foot. Actually, the torque is controlled by your right foot, but that translates directly into horsepower at a given speed.
Even though it takes 59% more power to go 70 instead of 60, that does not equate to a 59% reduction in fuel mileage, because power is a time function and mpg is a distance function. Sure, you're burning fuel faster but you're also traveling farther in that same time frame.
Just thinking about it logically, it seems like fuel consumption per mile should go up by the additional wind resistance over and above the scaling in speed. That additional wind resistance is one power factor, or 70/60 = 16.67%. There will be some other effects, too, like the increased frictional losses in the motor and potential changes in it's operating efficiency at the new rpm, but wind resistance should swamp all that. So if you were getting say 20mpg at 60mph, and you sped up to 70 and used 16.67% more fuel per mile traveled, your mileage would drop to 17.14mpg. Pretty close to what they said, 3mpg loss for 10mph change.
So I guess I basically agree with the article's conclusion, but I disagree with the reasoning behind it.
Maybe the article was a simplified explanation for us dummies.
Allison Jettester
08-05-2002, 13:13
For all you skeptics, I just read an article in the Aug. Popular Science mag where they did a test on a Silverado 2500HD Dmax/Allison and with some minor aerodynamic mods (tape on grill area and something to cover the area between the body and frame) and with cruise at 55 mph, got over 26 miles per gal (US). Seems that, in the same article, GM's unofficial overall fuel mileage estimate is 17.
Now if I could only keep from going to sleep at 55.
My first diesel was a 1982 2wd 6.2 overdrive with a 2.76:1 rear. Best ever was 29 mpg over a 300 mile trip. I could regularly get solid mid 20's. Even had a friend who put a 478 cube GMC diesel into a blazer with a 4spd. He got 20 on the highway. The only problem was the engine came out of a garbage truck and I couldn't tell in the morning if he was leaving or the garbage man was there!!!!
HOT DANG!!!!!!!!!
I JUST GOT 18.96 MPG. THE BEST I`VE GOTTEN IN OVER 21,000 MILES.GETTING BETTER WITH AGE AND MILES I GUESS. TALK ABOUT FUZZY MATH, YOU OUGHTA HEAR WHAT ONE OF MY FRIENDS THAT TRAVELS WITH US GET WITH HIS 'POSF' (PIECE OF SH#^* F#*+0).
UNBELIEVEABLE.
Jomar
woundedbear
08-05-2002, 14:14
Take a look for yourself:
http://www.popsci.com/popsci/auto/article/0,12543,332270,00.html
A crew cab, no less! Although not mentioned, it must have been a 2WD. These results will definately encourage me to use the winter cover once it starts to get the least bit nippy! Now, to fab some frame to body shields...
imported_
08-05-2002, 17:05
Fuzzy math?
doubt it
there are 3.78 liters in one US Gallon
there are 4.55 liters in an inperial gallon
i am using 1Gal = 3.78 liters
I get great gas mileage...
Too bad the truck uses diesel. :D
29mpg ??? Sounds like a typo or miscalculation in the conversions. Although, to validate some of higher mpg's received by some, one factor not mentioned yet is ALTITUDE. The higher the altitude the less "02" thus the computer delivers less fuel. Ergo better mpg. I have always gotten better mpg in the mountains than I get at sea level where I live. I'm averaging 15-16 empty city/highway.
just my 2-cents
txguppy
Somebody help me... The article said something about "taping the grill" Whats that?
Mike
Cruising speed of 105kph/65mph has given me an absolute best of 25.3mpg (Cdn gal), or 21mpg (US gal). Good thing this truck has cruise, or I'd be at 120kph on the highway all the time!
Overall average for truck's lifetime (mostly driving around town, unfortunately): 19.1/15.9.
Absolute worst: 15.4/12.8 (never had more than 7000# behind it).
'02 ext. cab D/A 4x4, 34k km on it now.
--Rob
[ 08-06-2002: Message edited by: ryeguy ]</p>
txguppy, there's a fixed amount of energy in the fuel. Going a given distance with a given load takes a fixed amount of energy. The only real way to change that relationship without changing the distance or the load is to change the efficiency of how the fuel is being used.
Altitude may not allow you to use the fuel as rapidly, therefore hurting power, but it doesn't really change the efficiency. If you have less power, it'll take longer to get the work done, that's the very definition of power. So you're just burning fuel more slowly for a longer period of time. No real change in efficiency, it still takes just as much fuel to do a given amount of work.
Notice how people with hop-up mods often report little or no change in mileage, or even a small increase? They're putting more fuel in and yet it doesn't hurt the mileage! More fuel = more power available = less throttle needed. Or, alternatively, they use the same amount of throttle and get the work done faster. Still takes about the same amount of fuel, because the efficiency hasn't changed.
What wipes out mileage is things that actually increase the work that needs to be done, like wind resistance or climbing a hill (although you generally get the lost energy back on the other side of the hill) or using the brakes. In particular, wind resistance is a big one because as you increase speed, wind resistance increases much more rapidly than the speed increase.
Colorado Kid
08-06-2002, 16:23
Ah yes, but the "wind resistance" (can I call it aerodynamic drag without offending anyone?) also decreases with altitude, in direct proportion to the density of the air.
Incidently, the Popsci article also did not mention which transmission the Duramx equipped truck had.
Their results are not hard for me to believe, in fact I expected even a little bit better, after all my best ever was 23.7 in my 2WD, ZF-6 Sierra 2500HD Crew Cab, with no "Taping", gap filling or bed cover, and that was doing my commute, which includes 80 MPH on I-25 north of Denver and Stop and go on I-25 in the "T-REX" project in Denver.
The last time I drove 55 MPH for a whole tank of gas was in 1987 in my '83 S-10 4x4 extended cab with a 2.8L (carburated) and 4 speed manual. That day I got 24 MPG running from Longmont, Colorado to Douglas, Wyoming and back to Cheyenne, WY.
As hard as it is to believe, I get almost exactly the same fuel consumption in this giant hot rod of a crew cab as I did in that little girble powered pop can. And I have records to prove it. Well, actually the worst ever for the D-max is 14.8 vs worst ever for the S-10 at 16.2 (delivering Pizza in the dead of winter), but the S-10 never pulled a 7,500# 5th wheel to Grandby, CO and back like the D-max did on it's worst tank to date. The S-10 also cost a very small fraction of the price of the new truck!
"Ah yes, but the "wind resistance" (can I call it aerodynamic drag without offending anyone?) also decreases with altitude, in direct proportion to the density of the air."
Excellent point ... txguppy, if you're seeing better mileage at altitude, that's a much more likely reason IMO.
bigbobed
08-07-2002, 15:14
Veerrry interesting. I always got better mileage with my Dodge V-10 towing my fiver when I was over 5000 feet. Felt like I had more power too. Thought it was the computer and fuel injection. This theory makes more sense.
Lots of interesting comments about fuel milage. I never would have thought of skid plates and under truck covers for better air flow. Sounds like the after market or manufactures should look into this. As for some stuff I see missing. No one mentioned rear end gear ratios. I've got 4.11's and for towing and low end great, but for a highway people mover not good. I've compared milage with a few friends and the lower numeric ratio's give great increases in milage especially at speeds over 60mph. Tire size can play a role as well. So when you friend says he's getting 25mpg and your getting 17mpg with what seems to be the same truck, compare a few more numbers.
[ 08-10-2002: Message edited by: yfixit ]</p>
DMAX/KTM
08-12-2002, 20:11
Wow, I don't know how you guys get such good mileage. I have a 2001 CC SB 4x4 2500HD Dmax/allison. I did a test a while back, had about 25,000 miles on the truck. 100% stock, 100% freeway, almost all level, drove no faster than 65 MPH, I even folded in both the drivers and passenger mirrors, took off the antenna, took off the tailgate, took it realy easy on the throttle. I was trying to break 20 MPG (US gallon), I was only able to get 19.8 MPG. I assume because it was hot and had the AC on? I have gotten 20+ MPG only once (20.2 MPG) and that was carrying a load of firewood in the bed of the truck but it was cold (no AC) and drove 60 MPH average. go figure. I kept track of about the first 90 tank fulls of fuel, always get 15.5 to 17.5 in the city and 17.5-19 on the highway no matter how easy I take it. Maybe it will get better as it gets more miles on it.
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