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.

 

[Diamondpilot]

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.

Are you noticing any loss of oil in that Cummins? Reason I ask is it looks like the filter may have a little oil soak. Maybe loosing it on shutdown with some sort of back draft on the turbo.

Chris

 

[mattman]

I have an 06 GMC diesel, I see about the fuel mileage you do maybe even a little less than you. Pulling my fuel mileage goes down like a rock. I have a bumper pull camper about 25 feet long or so and a small jon-boat on flat ground pulling them at about 60 say on average I get about 12 to 13 miles to the gallon. I pulled my camper in some hilly crooked roads and got about 10.5 to 11.5 miles to the gallon.

Right now I am running about 14.5 to 15.5 city and some hwy driving, with a little pulling of my trailer with a polaris ranger on it. I hear folks throwing out fuel mileage that just blows me away. I don't know how they get such good fuel mileage, my truck just does not get as great as some folks I guess.

 

[mattman]

On the how dirty the other guy's air filter is it all comes down to there the intake sucks it's air from. GM trucks suck clean air. By the looks of that filter Dodge needs to relocate their air intake to another location or do a better job of shielding it one.

 

[Hematite]

On the how dirty the other guy's air filter is it all comes down to there the intake sucks it's air from. GM trucks suck clean air. By the looks of that filter Dodge needs to relocate their air intake to another location or do a better job of shielding it one.

That's what I was thinking, too.

 

[Cami]

@WH401: Be honest, that's really a vacuum cleaner bag.

@radioman: Inlet restriction gauges aren't exactly precision instruments, but they're not all junk. If something doesn't seem right then double-check it with a manometer; if the restriction gauge is way off then junk it. Your dealer should have maximum inlet restriction specification (in inches of water) for your engine.

@mattman & Hematite: Amen! If your air filters are pluging up too quickly then there are a few things you can do like pre-cleaners/cyclonic separators but simply relocating or raising the air inlet a few feet can help. Those inlet snorkels on some large trucks aren't for fording...

Some thoughts about air filters:

The restriction indicators are important. Every time you pull out the air filter you let a little dust into the engine; it's even worse if you're doing the service at a dusty worksite. Changing filters too often means that you're exposing your engines to this more often than necessary. All that trouble light is truly good for is looking for holes/tears in the element.

I can't abide cleaning air filters with compressed air. You can put small tears or holes in the filter paper and, with dust blowing everywhere, some will get into the filter's clean side. If this is done near the filter box then even more harm is done.

The temptation to use air might creep into your mind when you're on a really dusty worksite where filters last less than a week and the costs start adding up. Fight the temptation and eat the cost of replacing them, add a pre-cleaner or have them professionally cleaned; up here in Canada, Finning offers filter cleaning services.

Actually, a filter cleaner is on my list of shame: The list of things I'd like to make that I haven't gotten around to doing. The idea is stolen from a friend who has what he calls his wall of shame; a bookshelf reserved for books he's bought but not yet read.

 

[WH401]

Are you noticing any loss of oil in that Cummins? Reason I ask is it looks like the filter may have a little oil soak. Maybe loosing it on shutdown with some sort of back draft on the turbo.

Chris

Nope, the dipstick reads the same each time I check it over the course of a 7500 mile oil run, and the inside of the intake tube is as clean as can be, no residue at all.

On the how dirty the other guy's air filter is it all comes down to there the intake sucks it's air from. GM trucks suck clean air. By the looks of that filter Dodge needs to relocate their air intake to another location or do a better job of shielding it one.

I'd have to double check to be positive, but the filter should pull air from just inside the front bumper, a location that's commonly used on most all diesel trucks today, specifically to keep away from the heat of the engine.



In all fairness though, the dirty filter in the picture is the Mopar filter that came with the truck, which has a natural yellow tint to the filter layers (those right off the bat give it a slightly "dirtier" look), and the new replacement filter was a Fleetguard, which is all white. My truck is going to be due for a change in the next couple of months at the rate I'm going so I'll have to get a picture of that filter as well.

 

[dodge man]

Let me get this straight, you are towing a moderate load at over 70 miles an hour and complain about the mileage? Nobody brought up the obvious issue, SPEED! Your driving too fast to get good mileage. The wind resistance is based on the square of your speed if I recall. In other words, wind resistance is 4 times greater when going 60mph from 30mph and nine times greater from 30 to 90. Once you get the load up to speed, the weight doesn't hurt you mileage that much, but the wind restiance of the trailer with a tractor on it is really high. Thats why they had the 55mph speed limit for several years. That said, drive how ever fast you feel is safe, time is also a vaulable assest.

Maybe you were driving a little faster with the lighter load because you felt safer, which hurt you mileage because of the greater wind resistance.

 

[jcmseven]

Dodge Man,

I think you make a solid point here. I tow to keep up with traffic, unless I am towing something really heavy. The interstate where I live is mostly 65-70 mph speed (mostly 70). If one drives 70, even in the right lane, people swing in front of the driver, run up on the driver, etc. To keep up with traffic flow, it becomes essential that one go around 70-75, which is generally where I tow. The truck has no trouble pulling at that speed and it generally tach's around 1800-1900 at that speed. Thinking that might be the "sweet spot" for towing, that is where I have kept the truck, but clearly it does not seem to like that speed. Without your having seen my trailer I can tell you it is an aerodynamic brick and likely has some role in the reduced mileage. That still to me, though, does not fully explain why my mileage dropped as is did. I was getting around 2-3 mpg better pulling the same load at the same speeds about 3 months ago. The truck runs great. What gives?? Is it truly more efficient to tow in tow/haul mode or in regular overdrive in this case?? Also, to me a load of this weight is a "light" load for this truck. Am I wrong here??

John M

 

[Diamondpilot]

I do not have all the answers. I would check the air filter again. If anything it may be installed wrong. Who knows. Also maybe it was a cheap air filter. My local GM dealer where I take my dads Corvette for service, he live in Cali and the car is here in Indiana, uses some of the cheapest stuff I have seen and run Valvoline filters.

It is probably winter fuel or they just got a bad batch. Like I said there is no standards for Diesel like there is with gas.

Also the sweet spot on the Dmax is 1600 rpms. The Fords that you are used to are around 1900 rpms. I pull a lot and with my Dmax I got my best economy at 62 mph. The same load behind my Ford 6.0's did the best at 74.

Chris

 

[RobertN]

When we went to Texas in August, we did pretty well going from Sacramento to Needles. Calif is a 55mph towing speed limit.

In Arizona and New Mexico, we bumped it up, near 65. About 62 was where I could really see the difference, watching the in cab mileage display. I know they are not necessarily accurate, but they give you an idea.

When we got to Texas, I bumped it to 70mph. The 5th wheel towed sweet, even at 70. But, my mileage fell apart like crazy. So, I eased back down to about 62-65mph.

I was getting 12mpg at 62 with my 01 Dodge/Cummins. That is with 4.11 gears(I would not do that again... If I get a chance, I would like to swap to 3:73...)

Oh, the other thing... No one mentioned tires. Unless you have stock tires, or recalibrated for bigger ones, bigger tires throw the odometer off, and in turn the mileage calculation...

Let me get this straight, you are towing a moderate load at over 70 miles an hour and complain about the mileage? Nobody brought up the obvious issue, SPEED! Your driving too fast to get good mileage. The wind resistance is based on the square of your speed if I recall. In other words, wind resistance is 4 times greater when going 60mph from 30mph and nine times greater from 30 to 90. Once you get the load up to speed, the weight doesn't hurt you mileage that much, but the wind restiance of the trailer with a tractor on it is really high. Thats why they had the 55mph speed limit for several years. That said, drive how ever fast you feel is safe, time is also a vaulable assest.

Maybe you were driving a little faster with the lighter load because you felt safer, which hurt you mileage because of the greater wind resistance.