Fuel Mileage

[WH401]

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...

I know the feeling, I have 4.10's in the 06. Running at 55 gets me the best mileage, unfortunately it also gets you run over on the interstate. If I switch to 65 or 70, which puts me at and over 2k rpm, the mileage drops off to around 13 - 15 mpg. I've found I obtain the best mileage at 63 mph, which is exactly 1900 rpm. At that speed I can get 17 - 18 mpg, possibly 19 but I've never gotten a full empty highway run. 63 mph also is a nice "compromise" speed for highway travel. Wind resistance and RPM are the two biggest killers of mpg. With the 3.55 rear end in the 92', I can get about 20 - 21 mpg on empty all highway runs at 70 - 75 mph.

 

[FlaDon]

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

Yes it is a light load but it about matches what a 2006 DM 2500 HD I know of gets.

 

[scoutcub]

OK, I have to bite and ask the silly question. Any headwinds? I just drove from MI to MA (around 660 miles each way) and as my luck would have it, I had a headwind both ways. Driving a F-350 Duallie diesel w/11.5' Lance camper pulling a 12X7 enclosed trailer. Knocked my mileage down 2-3 MPG from similar trips. At one point in the trip, I actually turned into a tailwind, and watched my mpg start walking up.....until I made another turn..... it makes a difference.

 

[jcmseven]

There might have been a headwind, but not enough for me to notice when pulling. I suppose as my dad says, "It gets what it gets" and I do not mean to beat this horse further to death than I have. All posts have been quite helpful and I have appreciated the expertise and insight. I am sure there is something I am doing or have done this time that is different from before and why my mileage has dropped off when towing. Interestingly, on my last tank, unloaded, I am back now up to my usual 18ish in combination driving. So far, I have made the following conclusions:

1. More power requires more fuel. To keep the DPF trucks at a high and competitive power level, the makers have had to increase the fuel loading.

2. Cool temps require more fuel. It seems logical to me that especially in a compression fired engine, this might be true--but I am not a mechanical engineer.

3. The two haul mode on these trucks is for performance and engine-braking, not for fuel economy.

4. Towing at highway speeds takes fuel. Interestingly, my buddy who has a similar 2007 truck to mine (post-DPF also) gets 12-13 pulling his 16000 pound 32' triple axle toy hauler, but he tows at 60-65, not 70-75.

I am planning to tow again in about a week and will back off on the speed. Will keep folks posted on what I find. For comparison (and I should be happy about this) my previous F250 6.4L got 9-10 with 3.73 gears pulling about the same load over the same route. It, too, started off better than dropped with about 12,000 miles on the odometer. Go figure.

John M

 

[WH401]

There might have been a headwind, but not enough for me to notice when pulling. I suppose as my dad says, "It gets what it gets" and I do not mean to beat this horse further to death than I have. All posts have been quite helpful and I have appreciated the expertise and insight. I am sure there is something I am doing or have done this time that is different from before and why my mileage has dropped off when towing. Interestingly, on my last tank, unloaded, I am back now up to my usual 18ish in combination driving. So far, I have made the following conclusions:

1. More power requires more fuel. To keep the DPF trucks at a high and competitive power level, the makers have had to increase the fuel loading.

2. Cool temps require more fuel. It seems logical to me that especially in a compression fired engine, this might be true--but I am not a mechanical engineer.

3. The two haul mode on these trucks is for performance and engine-braking, not for fuel economy.

4. Towing at highway speeds takes fuel. Interestingly, my buddy who has a similar 2007 truck to mine (post-DPF also) gets 12-13 pulling his 16000 pound 32' triple axle toy hauler, but he tows at 60-65, not 70-75.

I am planning to tow again in about a week and will back off on the speed. Will keep folks posted on what I find. For comparison (and I should be happy about this) my previous F250 6.4L got 9-10 with 3.73 gears pulling about the same load over the same route. It, too, started off better than dropped with about 12,000 miles on the odometer. Go figure.

John M

1. You hit the nail on the head, to a degree. The DPF's really don't cause the newer diesel's to have bad performance, they just cause the new diesel's to have to use more fuel, mainly during the ReGen cycle. During ReGen, typically extra diesel fuel is inject into the cylinders to achieve a 1200+ degree EGT, which burns off the built up deposit's in the DPF. This is one reason why it's good to drive the newer diesel's hard on a frequent basis, the more times you get the EGT's high without having to go into ReGen, the less fuel in will use, in a sense. A contrast to this would be if you put a programmer on a newer diesel, and then flogged it all the time. Even though your getting a high EGT, the excess soot is causing tremendous build up in the DPF and other exhaust treatment systems.

2. Very true. A diesel engine burns it fuel, where as a gasoline engine explodes it. The colder temperatures cause diesel fuel to not be burned completely, particular when the engine is still cold, thus giving you worse mileage.

3. Pretty much. On my truck though, I find I get comparable mileage while using tow/haul when empty because it allows the truck to have 100% power to the wheels without any slip of the converter, and it doesn't shoot the RPM's over 2k. I'm not familiar with T/H on the Allison, but what my truck does is it'll lock the converter at 27 mph in 3rd, wind up to 1900 RPM, and then at about 43 mph it unlocks, shifts, and re-locks almost instantly while going into 4th.

4. Very much so, driving slower helps a lot.

 

[dodge man]

A couple of other thoughts. Have you checked tire pressure on both the trailer and truck? In theory, cold air gives more power since it is denser, but it usually hurts mileage except on long trip, becuase when a vehicle is warming up it usually gets worse mileage. Also having the A.C. on or the windows down can hurt mileage too.

Another possibility is you just had a bad trip, maybe a little head wind, driving a little bit faster having the A.C. on alot, poor driving style, that all added up to poorer mileage.

You are wise to question the poor mileage, since in my experience if you see a noticable drop off, that usually means something is wrong.

 

[yooperdave]

Not to get too off-topic here, but my '06 Toyota Camry MPG's started to decline significantly in the last couple of months. I was getting 31-32 mpg, and now 27-28 mpg.

I drive 90 miles a day to/from work - mostly highway miles.

I thought it could be the amount of ethanol mixed in the fuel.

Now I am going to get me a new air filter to see if that will help.
I do not remember if the air filter was ever replaced. At 70K miles, it should be time for a new filter...

Thanks to all for the great discussion.

Yooper Dave

 

[om21braz]

If yours is anything like the Dodge/Cummins I had - winter & anything above 2000rpm really hurts the fuel mileage. Best true hand calculated with an 05 6 spd. 4.10 was 20.5mpg empty in middle of summer. Worst was winter towing approx. 6000lbs. = 13.5mpg.

 

[Reg]

You had it right there in your opening paragraph, "in the summer" you got 1/2 decent mileage.
In the Fall you got worse.
No surprise, other than you didn't know about seasonal fuel mix changes (-:

BTW, I think you're NUTS driving at those speeds, lightly loaded or not.
A trailer is basically a very unstable appendage to a truck.
THIS should be FIRST OF ALL !!! Your ignorance of seasonal fuel is a trivial non-issue.

I wouldn't care, but for the fact that other people are on the road - and I'm ONE of them.

 

[FlaDon]

1. You hit the nail on the head, to a degree. The DPF's really don't cause the newer diesel's to have bad performance, they just cause the new diesel's to have to use more fuel, mainly during the ReGen cycle. During ReGen, typically extra diesel fuel is inject into the cylinders to achieve a 1200+ degree EGT, which burns off the built up deposit's in the DPF. This is one reason why it's good to drive the newer diesel's hard on a frequent basis, the more times you get the EGT's high without having to go into ReGen, the less fuel in will use, in a sense. A contrast to this would be if you put a programmer on a newer diesel, and then flogged it all the time. Even though your getting a high EGT, the excess soot is causing tremendous build up in the DPF and other exhaust treatment systems.

2. Very true. A diesel engine burns it fuel, where as a gasoline engine explodes it. The colder temperatures cause diesel fuel to not be burned completely, particular when the engine is still cold, thus giving you worse mileage.

3. Pretty much. On my truck though, I find I get comparable mileage while using tow/haul when empty because it allows the truck to have 100% power to the wheels without any slip of the converter, and it doesn't shoot the RPM's over 2k. I'm not familiar with T/H on the Allison, but what my truck does is it'll lock the converter at 27 mph in 3rd, wind up to 1900 RPM, and then at about 43 mph it unlocks, shifts, and re-locks almost instantly while going into 4th.

4. Very much so, driving slower helps a lot.

#2 is close but not correct. Gasoline in an engine burns rapidly but doesn't explode normally. And both gasoline and Diesel engines can suffer mileage loss in winter. But a gas engine can also pick up HP due to increased fuel/air density.

As for #3, it depends on what you have taught the Allison. When new or after the battery has been out a while, you have to let it learn how you drive. Mine knows in T/H that if it needs, it can unlock the pump. Goes from 3200 RPM unlocked to 2800 locked in 4th at 65 MPH. In 3rd locked it turns 4K. Drops to 2100 in 5th which as far as I know, locked only. And in Grade Braking, I've seen it turn 4 K going downhill at 65 MPH.