Fuel Mileage

[jcmseven]

Fellow Posters:

I had a quick question about my diesel 2500 GMC/Duramax. It now has 18,000 miles on it and I have been very happy with it. I have been getting 18.5-19.0 mpg in mostly highway driving, unloaded, with a max of 22.2 on a long trip, comfirmed at the pump with hand calculation. During the summer and fall I towed my usual 8000ish pound load to my parents, a 3 hours one way tow which involves some steep longish grades and consistently got about 14 mpg (give or take a tenth or two) on this tow. The other day I took my dad my dad my 2320 JD tractor to use for mulch on my trailer. This load, trailer included, is only about 5200# as equipped. The tire pressures and brake adjustments on the trailer were checked and I had no problem towing the load (i.e., usual load). I only got 11.8 by computer and 11.4 by hand calculation, which included my return trip unloaded with empty trailer only. Granted, I am a fast tower, especially with light loads, so I had the cruise at 72-75 (in a 70 zone--not unusual) on the highway and ran the truck (as I do about 50% of the time) in tow/haul mode both ways. The truck ran and pulled impeccably but the mileage was less. Why would this be??? A couple caveats:

1. I am now running Rotella 5/40 synthetic oil v. previous 15/40 mineral based. I thought this should HELP the mileage.
2. I am now running white Power Service v. the previous grey container and this was the first tow trip with the changeover.
3. I fill up at the same station, no change there.
4. My tire pressures on the truck and trailer are unchanged.
5. I do not know whether this preceeded a regeneration cycle--the GM trucks are hard to tell when they are regenerating, where my 6.4L Ford made a big smoky event of it.
6. I was 10-20 degrees cooler than when I towed this exact load before and it was a cool damp day both ways.
7. I do not always use tow haul mode on the transmission when I tow lighter loads such as this, but I thought that might help the mileage a bit, not hurt it. I used it the entire trip this time.
8. I do not know if locally fuel dealers are getting in any special diesel mix for winter. I live in the consistently coolest part of our state, and it gets cold here early--I tell people where I personally live our temps almost exactly match those at my BIL's in Boston, MA. I do not know if that made a difference but other diesel trucks I have owned have all gotten substantially worse mileage in winter than any other season.

John M

 

[Diamondpilot]

Here is my experiences with diesels, owned 4. 3 Powerstrokes and 1 Dmax. Change your air filter.

Did I mention, change your air filter.

The filter may look clean and the filter minder may say its fine but the GM filter minder is junk. The Ford one is no better so not bashing here.

The other thing is the fuel. Diesel does not have a rating like gas, IE 87, 89, 91, ect. The Cetane rating, kind of like Octane, can very greatly and especially in the winter when many suppliers add kero to thing the fuel out and prevent gelling.

Also, did I mention, change the air filter. I think you will find this is your issue. You did not say what year your Dmax is but the 05 and earlier had intake troubles and with heavy loads, 14K or more lets say, and hot temps which you did not have they would overheat due to low air flow. I had this issue with my Dmax.

Did I mention, change your air filter. They are good, on average, for 20K. You are right in the ball park and its time. If its a late model, 06 and later, it will cost. The fix for the issues I had was a much bigger filter and they are $65 or more. Just for grins have the parts guy grab a 06 and a 04 filter and compare.

Chris

 

[Cami]

@Diamondpilot: I like the way you write. Repetition is an effective way to make a point memorable. Repetition is an effective way to make a point memorable....and the air filter is a darn good point.

When I have a hickup in my own fuel consumption figures, it usually turns out to be some situation where I did a fair bit of idling; once it turned out to be fuel theft.

There are all sorts of mechanical suspects when mileage takes a drop, but driving styles can have a dramatic effect as well. I was reading some findings from a fuel consumption analysis done for a trucking fleet that showed that for any given route, who was driving caused the largest variance; the most "aggressive" driver used up to 30% more fuel than the most moderate on the same route. I remember seeing that 30% and wondering if that driver was siphoning off diesel to run his oil furnace at home.

I'm sorry, but I can't remember the details and if I still have the paper it's burried in an impossible mess, so please don't ask for a citation.

 

[Hematite]

My local station has a sticker on the pump that says that the fuel dispensed from the diesel pump from October to April is Winter Blend diesel. If this is true in your area, this may affect your fuel mileage.

 

[WH401]

One thing to look at is winter fuel, it just doesn't burn with the higher amount MPG that you'll attain from summer fuel.

Secondly, the air filter change is a good point. In my 06' Dodge, I changed the air filter at 25k, even though the "minder" was barely off being perfect. Here's what it looked like, and what it looked liked compared to the new one:


I don't do any kind of serious off-roading, other than living on a 1/4 mile driveway, and you can see how nasty that thing looks. After changing the filter, I was instantly noticing an increase of .5 - 1.0 mpg at the pump, and easier turbo spool up.

 

[Diamondpilot]

That has to be the dirtiest filter I have ever seen. Where the heck you driving that Dodge?

Chris

 

[jcmseven]

Chris and others,

Thanks for the advice. My truck was bought in May, I never drive in dusty environments, and still had my air filter changed at my service at 16,000 miles. So, I doubt that is it. I used to run K/N air filters on my trucks but now I am big on factory spec ones. Unfortunately, in this case the air filter is clean. It could be the fuel, though. I have no way of telling that as best I can tell. My unloaded mileage has also dropped recently, but not by too much (averaging 17.8-18.0 in combo driving v. 18.5 ish before, so not too much a difference). I attributed this to seasonal fuel changes but did not expect it to affect pulling mileage this much--especially with a load my truck otherwise hardly seems to notice.

John

 

[radioman]

That has to be the dirtiest filter I have ever seen. Where the heck you driving that Dodge?

Chris

I was gonna say the same thing!

 

[radioman]

One thing to look at is winter fuel, it just doesn't burn with the higher amount MPG that you'll attain from summer fuel.

Secondly, the air filter change is a good point. In my 06' Dodge, I changed the air filter at 25k, even though the "minder" was barely off being perfect. Here's what it looked like, and what it looked liked compared to the new one:


I don't do any kind of serious off-roading, other than living on a 1/4 mile driveway, and you can see how nasty that thing looks. After changing the filter, I was instantly noticing an increase of .5 - 1.0 mpg at the pump, and easier turbo spool up.

I have a question for you guys. Why bother with the minder ? In my eyes, its just another doohickey that comes with the vehicle which is just basically a countdown counter? I always check my air filters with every oil change and use air to blow it out if a piece of leaf or sort of dirt fly up the air intake chute. If It starting to become hard to see the light but still see the light, I'll change it before the recommended interval. I believe the manual is there for the white collar suits who dont get thier hands dirty in the city and wants something on paper.

 

[WH401]

That has to be the dirtiest filter I have ever seen. Where the heck you driving that Dodge?

Chris

I was gonna say the same thing!

I know right, other than driving on my gravel driveway, (which is in pretty nice condition), all of my driving is on asphalt. How the filter ended up looking like that from the type of driving I do is beyond me. BTW, I stick with OEM filters as well, they're cheap and have great filtration efficiency.