Engine/Transmission (1994 - 1998) right gear ratio?

[jkh]

What is the best way to get the right ratio for what I want?

The sweet spot for my truck is right around 1750-1800 rpm's.

Trouble is, while towing the 5th wheel,those rpm's out of overdrive equalls about 45-50 mph( not enough for hiway travel) but the same rpm's in overdrive equalls close to 70 mph. I would like to be right around 60 mph at these rpm's in overdrive.

Any easy way to accomplish it?

 

[Skydiver]

What is your total weight? You have to be heavy to be having problems with your mods. Also what size tires are you turning?
I had the same problem when towing my (now sold) Newmar 5th wheel. I was 18,600 lbs total on Cat scales. With no power up grades I had my hands full pushing the OD & mystery switch buttons. I even tried running up to 80 mph going down hill when it was safe to do so.

 

[HBarlow]

What is the best way to get the right ratio for what I want?
The sweet spot for my truck is right around 1750-1800 rpm's.
Trouble is, while towing the 5th wheel,those rpm's out of overdrive equalls about 45-50 mph( not enough for hiway travel) but the same rpm's in overdrive equalls close to 70 mph. I would like to be right around 60 mph at these rpm's in overdrive.
Any easy way to accomplish it?

You are experiencing the limitations of the five speed transmission. There are two choices for improving your situation. See Gary Ames' (GAmes) posts on installation of a G-56 six speed manual in his '97. I think he wishes he had made the change years ago.

If you are pulling a heavy fifth wheel a 4. 10 gear set would also help for two reasons. It will allow you to run at a slower road speed in fifth/OD at the rpm you want and two, it will give you more load starting torque.

If you change the transmission and rear end ratio you'll have a new truck.

 

[CRWilliams]

Gear Ratio Calculator

An excellent page where you can try LOTS of different things with gear ratios and tire sizes. I used this page to decide I wanted 3. 73 gears with my 35's. Just have to get the gears and install kits now.

 

[SMorneau]

I always thought these engines were noted for harmonic torsional vibrations (aka torque spikes) in the sweet spot around 1700-1800 rpms. I lost several 5th gears in NV4500 in a truck that was driven in that range unloaded all the time. I am sure the TC will isolate a lot of that in an auto so it is probably not much of an issue.

Seems like there has been a lot written on keeping those RPMs up in the 2200+ range when towing. Yes, I know it sucks to have to do that and I don't think it help mileage any either.

I have already changed to a 5600 and if I had an auto I would keep it. As you know Dave G. can do wonderful things with them. If you must do something I would change gears before converting to a manual of any kind.

Scott

 

[HBarlow]

I've never felt or heard anyone else who has mentioned "harmonic torsional vibrations" in a Cummins inline six cylinder engine.

The 24 valve and HPCR engines produce peak torque from 1600 to 2700 rpm and peak horsepower at 2900 rpm. It is a low rpm torque producing engine designed to move heavy loads at 1600 rpm and above. It varies but the most economical operating range is around 1950 rpm.

There is no reason to operate at 2200 rpm other than to produce greater horsepower to avoid shifting down to a lower gear on a steep grade.

A Cummins will pull hills all day long at full throttle at 1600 rpm without harm but Cummins considers it engine abuse to use full throttle except briefly below 1600 rpm.

My first Dodge-Cummins was an '01 HO six speed dually. I ran it 325k miles before selling it to my daughter and son-in-law. I always (stubbornly) let it pull grades loaded at 1600 rpm (but never below) with foot to the floor. At 302k miles I had the NV-5600 overhauled because it had developed a bad bearing and was growling. The outer surface of sixth gear showed wear but was still quiet and working normally.

Some prefer climbing grades at higher rpm because they can avoid speed fall off and the need to downshift, others still operate their Cummins diesel like a car gasoline motor with a need to rev it.

 

[Bill Stockard]

I always thought these engines were noted for harmonic torsional vibrations (aka torque spikes) in the sweet spot around 1700-1800 rpms. I lost several 5th gears in NV4500 in a truck that was driven in that range unloaded all the time. I am sure the TC will isolate a lot of that in an auto so it is probably not much of an issue.



Seems like there has been a lot written on keeping those RPMs up in the 2200+ range when towing. Yes, I know it sucks to have to do that and I don't think it help mileage any either.



I have already changed to a 5600 and if I had an auto I would keep it. As you know Dave G. can do wonderful things with them. If you must do something I would change gears before converting to a manual of any kind.



Scott

Yes, the harmonic torsional vibration is what eventually causes 5th gear problem in the NV4500 transmission. The torsional vibration in a 6-cylinder engine is greater than a V-8 because of the longer time between fuel ignition in the power stroke. The NV4500 was originally built for a gasoline engine that produces much less torsional vibration since the compression ratio is less and the "explosion" from the igniting the fuel charge is less violent than in a diesel engine. Fuel ignition in Cummins HPCR engines is less violent because of the multiple injections.



You are also correct that the torque converter absorbs some of the torsional vibration. I think it may have an effect on the life of the torque converter lockup clutch with 12 and 24-valve engines.



I towed a heavy 5th wheel trailer with a stock '97 3500 dually with an automatic transmission and a 4. 10 ratio rear axle thousands of miles and understand OP's frustration. An axle ratio change to 4. 10 would help with his engine and transmission upgrades and would put the engine speed up to the 2000-2200 RPM range, depending on his tire size; however, it would be expensive since his truck is a 4x4.



Bill

 

[hsmith]

Mine likes towing at 2k a lot better than 1700 or 1800.

 

[NIsaacs]

The right gear ratio will always be elusive with the Cummins and only 4spd's.

This issue has been addressed by Dodge with higher rpm and 6spd trannys in the newer trucks.



I would say if you really want the right gear ratio, it will require a 6 spd transmission. An axle ratio change will still result in limited use on one end or the other.



Nick

 

[AClayton]

You are experiencing the limitations of the five speed transmission. There are two choices for improving your situation. See Gary Ames' (GAmes) posts on installation of a G-56 six speed manual in his '97. I think he wishes he had made the change years ago.



If you are pulling a heavy fifth wheel a 4. 10 gear set would also help for two reasons. It will allow you to run at a slower road speed in fifth/OD at the rpm you want and two, it will give you more load starting torque.



If you change the transmission and rear end ratio you'll have a new truck.

He has an Auto transmission.

 

[champspa]

at 1750 my 96 auto 3. 54 is doing 64 mph by the speedometer that means it's actually doing 61 mph according to my gps . I have stock wheels and tires . what size tires do you have

 

[AClayton]

I have the same exact problem with my auto and 3. 55's. Nothing you can do about it short of replacing your gearsets,and that is very expensive. I tow my heavy 5th wheel at 60mph in 3rd with my converter locked up at around 2150 rpm's... ... . have plenty of available power to tackle any hills without slowing. If I choose to go over 60mph and the road is flat ahead,I use OD and at 65mph I'm at 1600rpm. I realize that 4. 10's would be optimum for my situation while towing but the vast majority of my trucks use is to pull a little 6k lb trailer... which offers no trouble for me. I just tow my 5th wheel a little slower at 55 or 60 in 3rd lock-up and deal with it. I'm a little heavy anyway and the lower travel spds makes me safer on the highway vs. purchasing a dually with 4. 10's!!



Alan

 

[HBarlow]

He has an Auto transmission.

Oops. You're right. My first read and reply was early am before first cup of coffee.

Well if that's the case the only solution is 4. 10 gears.

 

[jkh]

Thanks for the feed back.

I did a little closer checking. No O. D. @ 1750 = about 42 mph.

O. D. @ 1750 =61 mph.

What I would like is to be at these rpm's at about 50-55 mph in o. d.

I pull about 10,000 lbs. 5th wheel.

Stock tires.

Goerends auto trans w/ triple disc(EXCELENT PIECE, by the way)

A gear between 3rd and O. D. is what I would really like (which I guess they accomplished with the six speed).

Is there a gear splitter you can buy for a 4x4?

 

[champspa]

looks like you need 4:10's to get that speed in od.

 

[surfbeetle]

Thanks for the feed back.

I did a little closer checking. No O. D. @ 1750 = about 42 mph.

O. D. @ 1750 =61 mph.

What I would like is to be at these rpm's at about 50-55 mph in o. d.

I pull about 10,000 lbs. 5th wheel.

Stock tires.

Goerends auto trans w/ triple disc(EXCELENT PIECE, by the way)

A gear between 3rd and O. D. is what I would really like (which I guess they accomplished with the six speed).

Is there a gear splitter you can buy for a 4x4?

If I'm not mistaken, you can get a Gear Vendors that is used only in two wheel drive mode for when you're towing on the highway but fits a 4x4 trans.

 

[cerberusiam]

Thanks for the feed back.

I did a little closer checking. No O. D. @ 1750 = about 42 mph.

O. D. @ 1750 =61 mph.

What I would like is to be at these rpm's at about 50-55 mph in o. d.

I pull about 10,000 lbs. 5th wheel.

Stock tires.

Goerends auto trans w/ triple disc(EXCELENT PIECE, by the way)

A gear between 3rd and O. D. is what I would really like (which I guess they accomplished with the six speed).

Is there a gear splitter you can buy for a 4x4?

You can't pull that weight at those rpm's and not trash the transmission, it won't take it. For towing you need to be over peak TQ and out of the harmonics range of about 1700-1900 rpm's. This is why its geared the way it is, to save the hardware. Adjust to run at about 2000-2100 rpm's in OD\lockup at your targeted hiway speed, thats the sweet spot for efficiency.



Use the lockup to adjust between 3rd and OD, locked drive and unlocked OD make the splits a lot better but you have to have the TC to get the TQ multiplication right or it flares too much.



No, the 6 speed just had a smaller jump from 1:1 to OD, still a crappy split.

 

[SMorneau]

The 24 valve and HPCR engines produce peak torque from 1600 to 2700 rpm and peak horsepower at 2900 rpm. It is a low rpm torque producing engine designed to move heavy loads at 1600 rpm and above. It varies but the most economical operating range is around 1950 rpm.

Yes, you are correct. Ironically in the most efficient RPM range there is a fundamental issue with the inline 6 design and the rest of the drive train. I have never "felt" them either unless you count the pain in my wallet:{ You can run full throttle all you want under 2000 rpms but I avoid it like the plague. I hope I can get 320k miles on my 5600.



Scott

 

[HBarlow]

Bill Stockard cautioned me about the power pulses of the mighty Cummins and advised keeping the rpm up a little when my truck was just a baby. I have great respect for Bill's experience and knowledge but I also have great respect for Cummins.

Cummins, Inc. , in a published operating manual of which I own a copy and in an e-mail response to me told me in writing that it is perfectly acceptable and recommended for economy to climb a grade at full throttle down to but not below peak torque rpm before downshifting. I have posted the quote here in TDR more than once over the years.

Of course Cummins folks were talking about the engine and not what that practice is capable of doing to the lighter duty six speed transmission used in our Rams.

The truth in my case was that practice did not cause failure of the NV-5600 in my '01 Dodge, a slight leak around the top cover and my carelessness in allowing fluid to get a little low then pulling a large, heavy fifth wheel from Moreno Valley to a coastal city on a day of fires in the mountains and stop and go traffic from Riverside County all the way across to the other side of greater Los Angeles. My transmission would have lasted 350 or 400k miles if I had kept it overfilled like I learned later from Standard Transmission.

My sixth gear did show surface wear at 302,000 miles and I assume that was at least contributed to by my stubborn insistence on climbing grades at full throttle in sixth gear.

 

[cerberusiam]

Cummins, Inc. , in a published operating manual of which I own a copy and in an e-mail response to me told me in writing that it is perfectly acceptable and recommended for economy to climb a grade at full throttle down to but not below peak torque rpm before downshifting.

That may be correct but it pulls a bit better and is easier on the drive train if you shift on the upside of the TQ peak and stay in front of it. Your numbers are off on the TQ peaks also, a stock CR will not pull well down to 1600 rpms when its peak is at 2500 rpms. Peak TQ on the 12 and 24V's shows right around 2000 rpms which is where the sweet spot is for towing. Under that its just needlessly lugging and rattling thru the harmonics band.



Auto, 5 speed, 6 speed, it doesn't matter, the harmonics kill EVERYTHING. :-laf