Poor fuel Mileage

[granduer]

I have. 2006 3500 dually with 5.9l, it has 373 reared and is 4WD. It has 66000 miles on it and my fuel mileage is horrible. New I was getting 15+ towing and 17-18 regular driving now I get 8.8 to 10 mpg towing. Nothing has changed same trailer and same load. I try to run quality fuel and Amsoil cetaceans boost and injector cleaner every tank. Oil change regularly. Feul filter change about every 10k. Please give me some ideas as to what going on with this truck.

 

[sisupower]

I have. 2006 3500 dually with 5.9l, it has 373 reared and is 4WD. It has 66000 miles on it and my fuel mileage is horrible. New I was getting 15+ towing and 17-18 regular driving now I get 8.8 to 10 mpg towing. Nothing has changed same trailer and same load. I try to run quality fuel and Amsoil cetaceans boost and injector cleaner every tank. Oil change regularly. Feul filter change about every 10k. Please give me some ideas as to what going on with this truck.

No tunes? Does it smoke bad?


Pro

 

[Mark31]

Is everything rolling free? Ever changed your brake fluid, greased the caliper pins, checked out the trailer brakes and bearings, air pressures. Shift into neutral at slow speed and see if it rolls to a "out of energy" stop or if you feel a slight drag as it stops. New, bigger, more aggressive tires?

 

[granduer]

Is everything rolling free? Ever changed your brake fluid, greased the caliper pins, checked out the trailer brakes and bearings, air pressures. Shift into neutral at slow speed and see if it rolls to a "out of energy" stop or if you feel a slight drag as it stops. New, bigger, more aggressive tires?

Everything has been serviced on a normal basis. Nothing seems to be holding back the truck, like brakes etc. same size tires as came on it.

 

[granduer]

No tunes? Does it smoke bad?


Pro

No tunes or tuner. No smoke. Do have just a very very small amount at idle but you really have to look for it to see it.

 

[TSimonson]

Did it happen all at once, or gradually decrease?

 

[granduer]

Did it happen all at once, or gradually decrease?

It seems to be something that started slowly. I thought nothing about but it has been getting worse over last year. More noticeable after going to dealer for airbig recall. It came back from dealer and noticed the interior lights acting strange and then found the tipm board was bad. Other than that nothing out the ordinary. The mileage was not as good before the dealer thing but now much worse.

 

[cerberusiam]

How are you calculating mileage and did you check the speedo to see if it was correct? What are you towing for weight? How fast do you normally drive DD and towing? What transmission?

15 mpg towing and 17-18 empty in DRW is, err, unusual to say the least. 9-10 towing close to max GCVW is pretty close to normal with no tuner and stock tire size. If you still have the stock exhaust with a CATand you are getting fuel smoke at an idle then look to injectors as the cause. 12 years and 66k with no extra filtration and the 06 injectors is potentially a problem.

 

[granduer]

How are you calculating mileage and did you check the speedo to see if it was correct? What are you towing for weight? How fast do you normally drive DD and towing? What transmission?

15 mpg towing and 17-18 empty in DRW is, err, unusual to say the least. 9-10 towing close to max GCVW is pretty close to normal with no tuner and stock tire size. If you still have the stock exhaust with a CATand you are getting fuel smoke at an idle then look to injectors as the cause. 12 years and 66k with no extra filtration and the 06 injectors is potentially a problem.

I am calculating mileage by hand. From fill up to fill up using the same procedure.

As far as mileage I have never gotten 15mpg towing anything. Even towing my 12’ mowing trailer I only get 12 at max. Towing my 14k gooaeneck with roughly 9k on the trailer I get at very best 10mpg. The last load from Alabama I got 8.8 with about 7k on the trailer. Only thing that has been remove is the cat. I have 4” exhaust from down tube back with a proformace muffle. When the truck is empty I get 14 to 15. That is down from 18 to 19 over the years. When I first got the truck is was getting 12 plus towing a 13K lbs camper. So I have no idea what wrong with this thing.

 

[cerberusiam]

New I was getting 15+ towing and 17-18 regular driving now I get 8.8 to 10 mpg towing.

First, I would suggest you go back and change your initial post to reflect correct info. You are the one claiming 15 mpg towing not us.

The mileage you have now is in the normal range albeit a little low. That could be due to age of the injectors but it isn't way out of the normal range.

 

[Mark31]

Grandeur, nobody knows your truck like you. I think Cerb is right---injectors. Softly put your hands on her heart when she's cold and see how smooth she is. Is her voice the same crisp note or is a little muffled? Does her oil smell/feel different? Get someone to gently drive her while cold and you ride behind, or, if you can get on a 4 lane, ride so you can see and smell her breath as she exhales. Are her shifts points at the same speed and smoothness? Treat her kindly through her illness---take her to a specialist instead of a gp-- and she'll be faithful to you. She hasn't had any OLD bio diesel to drink has she? Check her fuel filter close, and get her some extra filtration to protect her new injectors purchased from a REPUTABLE Bosch dealer. I'm trying to squirrel up some extra coins to buy my baby a set of BBI .5s. Mark

 

[slowmover]

Records matter. An app like FUELLY easiest to use.

I’d also note engine hours per fill. Average mph is key to understanding fuel mileage (time versus distance).

GPS correction of speedometer

Typical low mpg culprits (once service brought to date & miles) :

- Tires other than factory-spec closed shoulder highway rib.

- Alignment no longer to spec.

- On 4WD, steering slop

- Caliper drag

-CAC leaks

Seasonal changes that increase idle time (engine hours). It isn’t “winter fuel” that’s the problem, it’s idling, lack of preheat, and failure to use winter front.

On the trailer:

Brake & bearing preset
Tire pressure
Alignment
Incorrect loading
Incorrect hitch rigging.

The last is universal. Where the tow vehicle steer axle weighs same hitched or solo is the beginning. Use CAT Scale.

Usually, MPG losses are cumulative. Many small things adding up.

Failure to establish MPG baseline only makes things worse (same road course with and without trailer same speed where truck weight doesn’t change except trailer TW).