I have to admit, I am riding the short bus when it comes to Diesels.
I just bought a 2006 5.9L cummins SLT 4X4 this past weekend. I previously owned a 02 5.9 magnum and my gas bill was reaching $600 a month. THerefore I desided to buy this 06 for its fuel economy.......I thought.......I saw that people where posting that they are getting anywhere from 17-19 to 21-24 mpg......Well, I have been babing the H$ll out of her, one: to see what is the most extreme fuel mileage I could get and two: breakin stage. Well, she reads 14.9 right now with a quarter tank left. it is split 50/50 far as city highway....maybe more highway. Do these things need to significantly break in, or is something wrong. I have never pushed more then 25% down on the pedal and I live in S Florida where it is FLAT!....I did not ever come across a option with different rear ends. so I dont know it I got one made for hauling ang towing.....I am also a 2200 rpm's only going 70mph
Any help would be great!

 

I looked at the window sticker and "NO WHere" does it say anything about 3.73's or 4.10.......it just has the diesel engine option then says 11.50 rear axle and 750-Ampere Main.Free Battery tabbed under it

 

My diesel only got 14mph. I installed a bulldog programmer it's up to 17mpg now still not the greatest . But the power is AWESOME !

 

OK! Here is what I can tell you. I have a friend that bought his CTD the same time I did. I am getting 20.8 with 3000 miles on it. He gets 15 with roughly the same miles on his. After much discussion it came down to RPM's. My truck turns 1500 at 60MPH and 1650 at 70mph. He was turning higher more like your findings. He ended up finding out he had lower gears. He also found out is it expensive to change out. You can find out be lifting the rear of the truck and spinning the driveshaft. This enables you to get the ratio from diveshaft to wheel spin. It sounds like it may come out around 4-1. Hope this helps.

 

you have 4.10's

I have the NV5600 6spd and 4.10's and at 70 I'm turning 2300rpm. the 48RE has a slightly lower final drive than the NV5600 and a much lower final drive than the G56.

just to give you a little info, the auto (48RE) has two rear end options, 3.73 and 4.10, the G56 6 spd only has one, the 3.73 sincec the final drive ratio is rather tall. before DC changed over to the G56 the NV5600 had the choicee of the 3.73 and the 4.10's.

anyway, your mileage will only go up, I'm at 28,000 and my mileage is at an overall average of 15.8. but dont let that discourage you. when I pulled off the lot I was only getting 13-14mpg. now I'm able to get 19 if I'm easy. the best way to see the mileage increase is to tow something heavy and hard, or to put the 'wow' petal to the floor[8D] dont baby it, that is the quickest way to foul an injector. another thing that may help is a fuel addative many here have been useing with good results. Power Service (grey bottle) and Marvels Mystery Oil. 16oz of PS and 8oz MMO in each tank has given me a reliable 2.5mpg better over not useing it, plus with the new ULS Diesel that is comeing out the injector pump can use all the lube you can give it. anyway, you wont see the mileage go up untill about 10K. 20K is a good starter break-in period where the mileage will go up the most and level out.

These engines are just like a fine wine, they only get better with age

 

My friend's 04 CTD is getting about 17-18 mpg at 38k miles. He bought it new four door 4x4 auto Laramie model. He says it scared the crap out of him when he started out getting 13mpg. Since then, he got better at about 15k. Then again at 30k. He can baby it at 60 on the highway and get 20.5 but where's the fun in that?[8D] I have been talking to him a lot b/c I'm looking to buy one soon.


FWIW, he own's a mopar resto shop, and his partner has an ~00-01 RAM CTD with 175k on the clock. The shop owns a late 90's CTD with 285k on the clock. At 200k, they had their guys start to rebuild it. They pulled the bearings off and they looked fine, they just replaced what they took out to that point and put it back together. So basically, even if you get crappy mileage for the next little bit, I think it is still worth the investment, because you won't be making it again for a long time!

 

Consider yourself fortunate! I'm getting right about 11mpg average - sometimes as high as 11.3 ([:-]). Now, I'm running 4.10's and also running 35" tires, which puts me close, but not quite back to the same final gearing as a stock-tired truck on 3.73's. My truck has 4300 miles on it, and the mileage has consistently been around 11.

I'm sure if I kept my foot out of it I could hit 12. But I don't, and my wife is worse than me. Also, this is almost 100% city miles so far. But, even on the freeway it's not great because my leveling kit allows air under the truck and blah blah blah.

 

For what it's worth, I bought a 06 cummins quad cab in Feb. Pulled off the lot and the first tank was really dissapointing. Only about 15mpg. But, I've got almost 7K on the clock now doing mostly highway driving (turning 23-2400 rpm @ 70-75mph) and I now come in at just over 19 and rising. This is with the 6-speed, btw, not sure which gears it has off hand.

 

here is the gearing of the G56 tranny for those who are wondering. 3.73 rear axel is the only offering
1st - 6.29
2nd -3.48
3rd - 2.10
4th - 1.38
5th - 1.00
6th - 0.79
Rev -6.29

the 48RE which one can get either the 3.73's or 4.10's
1st: 2.45
2nd: 1.45
3rd: 1.00
4th: 0.69

for the older guys the NV5600
1st: 5.63
2nd: 3.38
3rd: 2.04
4th: 1.39
5th: 1.00
6th: 0.73
Reverse: 5.63

 

Good info there Drew. Thanks. I was kind of curious what 1st gear was considering the pulling power it feels like it has.