Its true, I need this truck to operate at the best efficiency especially at $4.10 a gal. You have two crowds that own these trucks; the guys that bought them for their shear dependabilty=mileage and the power guys=no mileage, but fast as a raped ape.

 

I'm getting around 18 mpg around town. The thing I've noticed is it doesn't seem to drop off when loaded. By loaded I mean, I'm usually carrying either 2-3 yds mulch, up to 1.5 ton top soil, 1 ton 2B, or between 1 to 1.5 ton of wall block. My milage seems to stay right around 18. Not that I haven't had an occational drop to 16 when I got in it all the time for fun and stress release. I did have a 15 mpg tank which led to the discovery of a severly leaking fuel return line. I've never calculated highway mpg, but, my best guess is 21-22 mpg...that's moving at about 70-75 mph...I don't do a lot of long, open highway runs. These mpg's are for the truck in my sig.

 

The conclusion I've come to for the 12 valve is:
4.10 gears = 12 - 17 MPG
3.55 gears = 17 - 24 MPG
Unfortuneatly mine are 4.10 and diesel is $4.27gal.

 

I would imagine 4.10's get better mileage in the city. And actually, there isn't too much difference between the gear ratios on the hwy either imo. It's more about speed.

 

When new, I got 21mpg consistently, now I get 17-18 consistently.
Never been able to find out what caused the drop. I just changed suddenly.
Maybe the fuel now is crap.
My speed averages 77mph typically.
In town, same mileage. Never see a difference.
A load of up to 2000 lbs doesn't seem to affect mileage at all. Pulling my 9500 lb trailer does, drops down to 10mpg. I drive about 65 with the trailer, at 73mph drops to 9mpg.

 

This happened to me a couple of times. One was a leaky fuel line, no problem. Other was tougher. Head gasket was broken, and couldn't see a problem until it had been broken quite a while and was bigger. Got that fixed (well, head was spalled too, so it needed machining) and the mileage went up instantly.

 

Last 2,600 miles my 305/555 has averaged just above 19 mpg, all in town since January first. That looks consistent with some 12V numbers I'm seeing on this thread. I keep it between 1600-1900 for all driving. There are others doing as well with '03-'04 trucks.

 

Actually on the hiway the Auto's overdrive is a higher ratio. Than the 5 speed. like .076 I think.

 

i have a 97 2500 6spd 4x4 thats gets about 14-18 and just put my frist tank though my 97 3500 5spd 4x4 and it was 15 both have 4.10's this is right about the same that my dad gets out of his 06 with 3.73 and a G56 now that he has the bulldog on it

 

To be more exact, it comes down to rpm's. The sweet spot for the cummins seems to be 1750-1850, from my experience.

 

With the 3.54 rear end and 5 speed the torque is poor until about 1900 rpm, then it is suddenly great up to where it defuels at about 2400 rpm.
The automatics seem to start at about 1400 rpm or so, much more useful range, different pump adjustment I guess on this engine.

I blew an intake manifold gasket and it really lowered the mpg til I finally found that problem. Thats happened twice.
The dealer blew it on test drives each time. So, I don't EVER go to the dealer now and things are fine.

 

One reason some people have seen an economey drop may be the low sulfer fuel we have to burn. Don't know this for a fact just kina wondering.

 

I've got the 215 pump engine, so i'm good starting at around 1500 rpm, never put it in 5th until 1700 though, keeps the NV4500 happy.

 

that is somewhat true, i usually get around 13.5mpg and thats hitting 30psi on the boost gauge, many times in one day, sootin quite a bit, and driving fast, if i go slower, it goes up to 15, but my tranny slips a lot now, before it was slipping i was able to get 20mpg if i drove it really easy. and diesel is now 4.50 a gal so im parking the truck within a week and getting a little 500 commuter car, so i don't have to drive my truck everywhere. but i'll still probably drive it on the weekends.

 

"im getting 27mpg mostly highway some city. but i drive to save fuel, speed down hills coast up hills stuff like that."

sorry but I'm calling bs on that one...maybe on a calculated 1/4 tank of mostly downhill......