Have 99 2500 24v with 4.10 4 speed auto. I pull a 30 ft fifth wheel. I have talked to a couple of different cummins owners who also have auto trannies who say they pull with od locked out and run 65 mph at about 2500 to 2800 rpms. One of them said they also were running 4.10 and the other said he was running either 3.54 or 3.73, he had a 2001 dually. I tried locking out od and at 2600 rpm could only get about 52 to 53 mph. I'm considering regearing rearend but not sure what is available for a 1999. Does anyone one have any experience with this or thoughts that would be greatly appreciated.

 

Keep your 4:10's for towing, you will be disappointed with the 3:54's towing in OD and OD lock out brings the rpms up too high for decent fuel economy. The 4:10's are also easier on the auto transmission, help it to last longer.

2002 year model and lower 24 valved trucks all had either 4:10 or 3:54 gear ratios.

 

I tow my 31 footer in OD....I have no problems...i just either speed up before a hill, or i slow down and kick it out of od...

 

I use the O/D lockout button a lot. I take off with the O/D off and when I get to about 2200rpm in 3rd I push the button and run in O/D. I just watch how the tranny and truck is operating and use the button when needed. I discussed my 'technique' with Dave Goerends and he saw no problem with it.
Tom

 

With a 4.10 rear you should rarely have to lock out the OD. I calculate 2000 RPM to be roughly 65 MPH in OD. What is the problem?

4.10 is the only acceptable towing gear (if not lower) for an auto IMO. I had 3.73 and it was tough to get loads moving and keep them moving.

 

Really appreciate you're response to my questions. You're calculations are pretty close to my situation in that when towing in OD @ 65mph I'm running about 2000/2100 rpms. What I was looking at was when I encounter a incline or hill even at 65 my rpms start dropping off and while giving more throttle they continue to drop off. I can drop out of OD but have to wait until I'm below 50mph or my rpms jump close to 3000/3100 rpms. Besides losing most of my speed I'm also concerned about dumping fuel into cylinders that isn't being used completely. As you may be able to tell I have towed quite a bit with gas engines but this is first experience with diesels. So I'm learning.

 

If you are running those kind of RPMs you can put your foot to the floor and not have to worry about excess fuel. As far as the comment of 3.54 not being any good for an auto, I can get going loaded to about 17,160 (scale weight) and a friend of mine has a hard time keeping up with me in his Escort. I need to get going like that just to keep the RPMs up so the tranny don't up shift to soon. I still see about 12 to 14+ mpg towing.
Tom

 

For me with the 3.73s, and slightly larger than stock wheels/tires, 15,000 gross was a breeze, but it was when I went up to 21,000+ that I didn't enjoy it. Not to say that the truck couldn't do it but getting going was a hassle and so were hills... I would have much rather had a 4.10 rear end.

 

Running that kind of weight I can understand the 4:10 gears. For the 30' fiver, there would not be any real advantage with lower gears.

 

Your also a long ways from stock. Even when I towed 15,000 lbs with the built up 99 with 3:54's I had it would slow down on hills unless I turned the comp box way up.

cartrl
I wouldn't change anything on your truck unless you want to start doing some serious upgrading of parts. If you run in third or OD lock out with 3:54 gears you will be buying alot of fuel driving at 2700 rpm's.

 

I have an auto in my wifes truck and pull max about 10k. I pull that in OD with 3.55's. I have same gears in my sixer and the only reason I am going to 4.10's is because of the 35" tires and I pull 20k often. I would go to 3.55's so when pulling up hills and you are slowing down you can shift out of OD and not red line the **** out of your truck. 3.55's will pull a hill faster than 4.10's out of OD. You will be able to hold a hill longer in 4.10's before it kicks out. But the way our trucks are set up, I usually dont have to lock out, or shift out of overdrive in the mountains up here.

 

Why thank you!! All I do have is a (ahem) modified #8 plate and the timing at about 16.5* I hope to dyno next month to see where I am at HP wise.
Tom

 

My bone stock '01.5 auto/3.55 was ok running 65-70mph. But it was real sluggish at 55-60mph in O/D. I could run 58mph at 2300rpm with O/D locked out but it was kinda noisy. When I bought my '03 S/O with the same power output, I opted for the 4.10 ratio with automatic. It was a better match for the power band of the motor in O/D from 50mph to 70mph. on steep uphills I had to slow to 58mph before it would allow a downshift to 3rd. Not really a problem but I would have like to be able to maintain 65mph uphills. Overall, I prefer the 4.10 with the 2nd gen. I would imagine the 3.73 would have been the better choice for the '03. A good compromise...
I think you should keep what you have. You could upsize the tires from the stock 245s to 265s and increase the speed you can run in 3rd to around 60~mph.
Dodge should have beefed up the 545RFE for the Cummins. The 3:1 1st gear and double O/D, .75 and .67 4th and 5th gears would be great with the Cummins. I don't really reap the benefits of it with the Hemi. I run in 3rd most of the time towing.