Truck specs

fuel mileage is king of the specs! dont settle for 6.5mpg when you can do better. i reccommend shooting for 8mpg+ if you are hooked to a flat.

 

3:42 is low for an ACERT "twin turbo" CAT. 3:36 and 3:25 are normally what you see spec'd with an ACERT. They operate more efficiently and pull better at lower rpms than the older single turbo MBN's, 6NZ's or E models. If you have a single turbo CAT 3:55's or lower 3:70's are ideal with a 13 speed. I'd get better fuel mileage at the speeds I run with 3:55's but it sure is nice going up the mountains loaded to the gills 10-15 mph quicker making better time than others. 3:70's if I drove 60mph would have me over 7mpg even without the benefit of a roof fairing on my T600 pulling this dry box. As it is I avg low 6's. 3:55 would probably suit most better but I like my setup. Another key thing get in touch with Mr Haney in the garage section so you can get your ECM tuned properly. In stock form before his ECM tune and other recommended upgrades my motor would average 5mpg on a good day with zero percent idle time and conservative shifting habits.

 

I agree with the rear end change except i would go 2.64 or 3.08 out in Texas you can run 75+ need your target cruise rpm about 1200-1300 for mpg's, won't be the first to the top of the hill but you'll be one of the last ones to the fuel pump if driven "correctly".And as always a Haney tune never hurt anybody,(except their cheeks from grinning so much)

 

First thing you should do is get it into Mr. Haney. He will turn that motor into a monster, if you want.

 

since we are going older truck I would look for a '03 or older truck with a series 60 or a cummins N14 with either 2.54 rears and a direct drive tranny or 3.55 rears and a overdrive. Lo pro 22.5 tires are your best choice for for fuel economy. Reason for the 03 or older is it will be pre EGR and much more reliable. If you keep it around 60-62 and don't get 7+mpg with those specs there is something wrong with the truck.

 

RizenPhoenix brought up a good point with reguard to the gearing, if you are going to spec 3.08 or 2.92 rears with 24.5 rubber then you need to have a direct drive trans or if you run a 13 or 18 speed with double od specs you will want to run in the final direct drive gear (11th or 16th) because overdrive will be usless when you are loaded heavy or even at gross wieghts of 65,000 the truck would struggle in od. As for engines the 04-07 years will be built with the low knox emmisions requirments and do not perform nearly as well as the 03's and older ones did out of the box, mpg will also be worse on the later engines because of this and there isn't alot you can do about it without some pretty significant changes in the ecm and some hard parts as well. also remember that your company truck is electronically being held back by the company which is very common and so you shouldn't completely judge all trucks based on how this one is performing.

 

Here is a comparison of these ratios at the very same road speed:

Gear 11th 12th 13th

Ratio 2.64 3.08 3.55

Ratio 2.79 3.25 3.70

Ratio 2.47 2.93 3.42

The advantage of running with something like 3.08 instead of 3.55 is that you would stay in single overdrive for moderate speeds and go to double if you need to step it out and still keep rpms low for mpg. Running in single over is less parasitic of hp and torque making it to the drive wheels than running in double over. And for those drops on a hard pull, going down to 11th, direct drive, will maximize all power available to the rears. You sacrifice hardly anything in start ability. I agree that that the Cat likes lower rpms for max fuel economy. And they pull down low pretty well.

It is a common misperception that running a little taller rear, you sacrifice power. Not true. The more you can stay out of overdrives the better for performance and economy. Eaton has proved it time and time again, and it is a matter of published fact. You sacrifice more power and efficiency in overdrives. Now, it is not practical for some folks due to what they haul and where they do it to run something like 2.64 rears for maximum benefit (like I do with an 18 speed in front of it). But at least going to 3.08 would reduce the parasitic loss of double over by using single over most of the time. And we all have to "kick it in the rear" occasionally. Having another gear up to move to and being able to keep those engine rpms down at faster speeds is icing on the cake.

Does it work? I am having no problem pulling 80K gross around. Even on very hilly, winding, two lane roads with 20 mph corners at the bottom with pretty impressive grades coming out of it. And my truck is averaging mid 7's on fuel mileage. Got a couple of nice, lighter loads this week and came out of the week with a pump to pump mpg average of 8.3. Not sure what the ECM is showing, since I do not have an ECM fuel mileage readout on the dash. So, these are actual numbers from real pump to pump calculations.

Will it work for everyone? No. It does depend on the operation. If one is doing serious heavy haul, running in and out of rock quarries, coming out of soft cornfields with 120K gross, etc then would not be the best choice. But if one is mostly highway running, pulling typical loads, then it can be a very good choice to consider. And for gross that you mention, something like 3.08 would do just fine for what you are desiring.

 

It only take 200hp to maintain 65mph..... The big HP is for getting you TO that 65 or hauling heavy.