This past Saturday I was given the oppurtunity to drive an '08 3500 4x4 dually with the 6.7 and 6 speed auto. I enjoyed it so much that I am prepared to part with my 06 5.9 6 speed manual truck.

Now, before I get a bunch of nay sayers, please hear me out first then let me know if it's a known thing.

The owner complains of only getting 14 mpg ever as his best. The truck is always unloaded. My trip was around 215 miles total. Filled up before and after and tried to fill as similar as possible. Slow fill to first click off, and not squeezing more in. The trip was mixed highway, backroads and city/town mix with maybe 40% highway. I never went over 65 on the highways and stayed around 45-55 on the backroads. I used the exhaust brake on all the time and spent most of the time using the auto stick feature. I stayed between 5 and 6 on the highway and mostly on 4 in city/town. Leaving it in D seemed to let it go into higher gears alot sooner than I would have with mine. The rpm's would drop to 1100-1200. Now I know in my experience, that that is too low for a Cummins to operate without the computer loading the motor with fuel to light the turbo and prevent it from stalling out. I do it in my current truck to smoke out pedestrians or intersections at will. I could actually hear and feel the motor lugging along this way.

I started getting aggressive with the auto stick and kept the rpm's at 1500 or better at any speed. The truck felt really peppy and it wasn't lugging anymore. The overhead started to climb. I did hit reset after I filled up the first time and it stayed around 15.2-15.4 once it settled. Through out the rest of the day it managed to climb as high as 18.4 on the highway. It was a humid day so I did alot of idling and used the a/c too. At the second fill up (around 12 gals) before we were done I got a hand calculated 17.8. Overhead still read 18.2.

So I've got say I was very impressed with the new tranny. It was just like my truck but without the clutch. The ex brake slowed the truck and the extra gears helped to move it along quite well.

So yes, I know there are dark clouds forming over my parade, but if any current owners are having this kind of experience I'd love to know. Better yet if anyone is doing exactly what I was doing and it isn't getting you positive results, I'd like to know as well.

Thanks,

Chris

 

I tried driving, as you did, in 4th and 5th for several days. Throttle response is much better while driving this way but in the end my mileage had actually gone down about .5 mpg. Not major but at $5 per gallon I went back to driving with it in 6th gear. Unfortunately if you do that too much then you may soot up the turbo and O2 sensors, which I did. Then you have to drive like a maniac, while obeying traffic laws of course, to clean things back up.

As far as highway driving, I found that mine gets the best at 65mph exactly. Doing 70-75 mph makes my mileage drop quick so I believe your truck owner's story. If he would have just slowed down he would see the numbers you did. I have gotten a best of 20mpg if there are no big hills. Generally I get 17-18 on the highway. Well, that all holds true until your truck goes into regen and then you can watch the mileage drop at least .5 to 1 mpg until it's done.....

Other than the mileage the new truck is awesome. Try running it in Tow/Haul with the E-Brake on and in Manual 4th gear and come down a long hill. It is truly awesome how well that truck will slow down without you touching the brakes....

 

62-65 seems to be the sweet spot. I was on a long trip today and needed to make time, so was traveling on cruise at 70-75. Looks like I got around 16 when I usually get 18 on the highway at lower speed.

The toggle shifter is nice, but I find I don't really use it. Although it does drop down to 1200 rpms at 45-50 mph, it doesn't really seem to bother it much. I just stick it in D and go (after engaging the EB, of course). The 6 speed auto is definitely much better all around than the old 4 speed. I was sold after the test drive and traded my 2004 for the 2007.

If you are prepared to accept that the fuel mileage is about 2 MPG lower, you'll love the truck.

 

Back in January I traded my 06 3500 SLT DRW Quad 4x4 with a 5.9 and the 6 speed for a 08 Larime 3500 Mega DRW with the 6.7 auto and I love it would not trade back for anything, I only drive it about 3-5K a year so the small loss of MPG doesnt affect me. I say go for it.

 

Glad you got to drive one first before buying one. I did as well and love the truck. Mileage is not that bad for me, I just went on a 700 mile this weekend trip and got around 21-23 mpg doing 62 mph.