I have a 07.5 6.7L 3500 4X4 Dually, and I'm towing a 12K 5er. Usually I stay at 63-65MPH and my RPM's are 1600-1650. As my truck is stock and do not at this time want to modify, what do you recommend I keep the RPM's at to be in the sweet spot? Or am I in it?

Thanks

 

Optimum rpm for this engine is between 1450 and 2100. You decide depending on conditions.

 

I ran for over 80K miles hauling campers at that rpm in 6th...final gear. I did drop down to 5th manually as soon as I started up any grades and I also would drop down manually if I got below 60 mph and planned to run between 50-55.
It seemed like the trucks automatic trans would down shift just a bit too late in the rpm range when towing for my truck.

 

Tough question, Do you have boost and pyrometer gauges?

If you can keep the boost below 10psi and the EGT's down and rpm's below 2000 that's the best overall. You must also consider terain. If you get into the hills it's hard to balance mileage and EGT's. I get lower EGT's with more PRM and boost but the MPG goes out the window.

 

my 01 with 3.54 tows best in 5th it will tow in 6th but EGTs run at 900* on flat land so I kick it down to 5th and 2000-2100 and my mpgs don't seem to change but my boost is lower and EGTs are at 700* .if i have a good tail wind I will kick it in 6th and run 65mph and it seems to do good .but like was said you need to find the right spot for your driving -load

 

This question has been the subject of debate for years. As an example of this, put your feet up, grab a drink and start reading the following and similar threads over the years:

https://www.dieseltruckresource.com/...ght=sweet+spot

 

Im currently traveling the USA with my family towing a 15,000 lb fifth wheel with my dually and to me in 6th gear with my 3.73 gears 62mph works but the EGTs get high pretty quick if pulling any grade with it in 6th. The sweet spot I feel is 1750 rpm so in 5th gear im doing 55mph but if im in 6th 1750 would be close to 70mph which doesnt help with mileage due to wind resistance. Traveling in 5th @ about 60mph and rpms @ 1850 while pulling a hill my EGTS go to about 1200 degrees and pulls the hill. If im in 6th doing 62 mph @ 1600 the EGTS pulling the same hill will climb into the mid 1300s and you can feel the truck lugging. My observation with my truck and setup is 1750-1850 is the sweet spot. Just my .02!

 

Here is a suggestion for all the 6.7 guys with 3.73's. With the 68RE, in my opinion, 6th gear is way to low in the RPM range for towing a heavy fifth wheel.

So give this a try. Run in 5th gear at 65 mph which will give you a 4:88 ratio and you'll be turning just under 2200 rpm's. Cummins says that you should avoid while towing or in a loaded situation, all rpm's less than 2150 at your target speed. Say 63-65 mph.

See http://www.powerspec.cummins.com/sit...ng/gearing.htm

I have a friend with an 08 3500 6.7 with 3.73's. He has no monitors or guages in the truck. He has done a complete Delete and added a Smarty Jr at the highest HP and Torque rating. He pulls a 37ft 16,000lb Carriage Cameo.
His truck has the instant mileage read out like the 00, 01 and 02's had. I suggested to him to try what's above and he did. The overhead read out shows an increase in mileage right away. That only means there is less load on the engine in 5th gear. He has not actually run in 5th gear for a tank of fuel to find out what he's getting, but admits the truck runs much better in 5th while towing than 6th. He said he had a lag in 6th until the turbo comes when running 1600 rmps in 6th.

So with the new 800 Cummins coming out, I see Dodge is using a 4.10 gear with the heavy duty tow package. This, I suspect is to get the rpm's up in 6th gear. It won't be enough for the gearing Cummins Recommends but it will be better than 3.73's by 200 more RPM's.

Dave

 

Dave, I total agree with the 4.10s for optimum towing gear! I weighed the options of a gear change more than once before our trip but decided to go with what I had. I tried running 60-65 in 5th gear for the 1st couple hundred miles of our trip but the rpms were around 2000 and it seemed to drink more fuel. I really like the EGT gauge I installed, its the best gauge for me to watch if the engine is lugging. Once it starts to lug the EGTs shoot way up to 1200-1300 rpms! I then can decide if its time to down shift.

 

Chris;

The smarty Jr is supposed to reduce EGT's. ?? So its advertised. I run the Juice with Attitude and never see anything over 1150-1225 EGT's even pulling the Mississippi River Bridge at Baton Rouge at 64 mph with the Edge on tow position. Truck never slows down just runs the boost up and never down shifts. If I could improve my gearing I could run the Drive Position on the Juice. 385hp 780 on the torque. Next gear up for me is 4:56. I currently run
just under 2000 rpm at 65 with 4:10's an get 10.5 to 11 mpg at 23,400lbs combined.

My friend, I mentioned with the 6.7, keep telling me about the Smarty Jr and the reduced EGT's. I asked him how do you know that? I would pull the box out and go back to a stock computer and see what happens with the EGT's.

I'm almost back from a 4000 mile trip since March. Currently in TT Orlando.

Will be headed out again the first of August forever as I hope the house will be gone. Will be in Bald Mountain for a The Dixie Traverlers, Carriage Travel Club Campout on August 12-14th.


Dave

Carriage Travel Club #17771
Florida Road Runners Chap 54

 

Dave,
Thanks for the info. My truck is stock, without gauges etc. I had always thought the higher RPM's say above 2000 and in a lower gear, would use more fuel and heat up the trans. If the consensus is no harm to the trans and better efficiency engine wise, I will try your recommendations and see what the results are. When towing I generally stay in 6th gear anyway.

 

When running at 65mph with the outside temp at 96, my tranny is right at 204-206. I slow down to 63 I can bring it back to 196-197. I try and keep it under 200 when its cooking outside. Change the Tranny fluid more often. If you don't have a Tranny temp gauge, figure ambient temp plus 110-115 degrees at 85 and up. Below 85, its running cool. Oh yea, when headed into the sun the tranny temp will come up as the front of the truck is heating up.

I don't have any experience with the 68re so temps could be a little less or more. I would at least get a monitor from some where that reads want your computer is seeing. 5 minute installation. Just plugs into the computer terminal. 300-350 bucks.

My personnal opinion about extra fuel at rpm's around 2000-2200 is the engine is running with less load and should do just a little bit better. Its the barn door behind the truck that's eating the fuel. So,, a combo of less speed and more rpm's (better gearing) should give you better mileage.


Dave

 

I just got back from a trip to St Louis Mo, pulling our 5er down for a long weekend. I pulled both ways in 5th gear, locked out 6th gear and did not use tow/haul with the cruise control on. The RPM's were between 1850 and 2000 and I was towing at 60MPH mostly. My fuel mileage 12.4 MPG for the round trip of 710 miles.
My truck is equipped as follows; Smarty S67ME sw3, 68RFE, 3:73 gear set, 4X4, running LT265/70R/17E Michelin M/S2 tires.
Jim W.

 

I wouldn't put much stock in the PowerSpec ratings because they assume much higher loads then a Dodge pickup can really show the engine.

In other words, the recommendations have a built in bias towards performance over fuel economy. If you follow their "performance" recommendations, you'll have a RPM screaming monster that can take any hill but will get single digit towing economy.

There's a really simple rule to figure what towing RPM is "ideal": it's the lowest engine RPM that still delivers the required torque demanded by the load.

The 2010 6.7L engine has basically flat torque output from 1600rpm to 2200 RPM. If you wanted ultimate towing performance, you'd cruise at 2200 RPM and have 600rpm of lug down capability at constant torque with no downshifting.

Downshifting kills fuel economy. A lot of drivers think they are achieving fuel economy by specifying taller gears, but if that means you are downshifting a lot of times in the hills, you'd be far better off in many cases with a shorter axle gear that lets you stay in OD.

So gear for a cruising RPM that reflects your driving speeds, terrain, and average load.

If you are loaded to max GCVWR, and tow in hills, then you want to shoot for no less than 2200rpm cruising. If it's one or the other (but no both), maybe 2000-2200 is a better choice. If you tow only 5K-10K in less abusive terrain, then go even taller-- maybe 1900 rpm cruise speed.

If you are almost always empty on level ground, then you want to be close to that minimum peak torque speed of 1600rpm. Figure 1700 or so as a cruise speed.

JH

 

I sure we all appricate your input. Please jump in more often.

The one thing, I think your not taking into consideration is the transmissions that are used in the dodge trucks. In my case a 48re without lower gearing would not hold the load at my combined weight of 23,400 lbs for very long especially if I still had the 3.73's. I wanted 4.30's but no one makes them.

That still does not give me a screaming engine at 2000 rpm's at 64 mph. The power spec web site may be intented for trucks of 54,000lbs GVW but you can be assured, the tranny in there will hold the load.

The only way to make sure my tranny stays together is to keep the load factor down. So in my opinion the power spec site applies to anyone that tows at max weight. In my case the load factor shown on the Attitude with 3:73's was 76%. With the 4.10's it was 54%. When I go full time, and do nothing but towing the fiver, I intend to change gears again to 4:56 which should drop the load factor again. 4:56 gears will run 2100 rpm's at 63 mph.

I could go the other way, if someone made 3.23 gears, which would allow me to tow at 2200 rpm's at 64 mph, in 3rd, and run the highway at 1600 rpm's at 68 mph in overdrive. Best of both worlds. 3.48's can be had and I might look at that again as there is only about 100 rpms difference in the two gears when towing and running the hwy. Another tranny cooler may be needed in my case based on what I've seen with my rig when towing at 96 degrees outside.

Like I said in a previous post on this tread, it's the barn door 13ft in the air 8ft wide on the Fifth Wheel thats giving all that load at speeds higher than 55 mph.

Dave