Just wondering if anyone has come across a hp and torque curve from a dyno for the 305hp engine in the 3500 CC. Interested in seeing what rpm gives peak torque.

 

Go browse over at Cummins website
http://www.everytime.cummins.com/eve...ab_ratings.jsp

 

Thanks, that is what I was looking for.

Peak torque hits at about 1600 rpm. Does that mean for max economy I should get a gear/ tire combination that give me 1600rpm at the cruise speed I want. If not what rpm should cruise rpm be for max economy.

According to the spreadsheet by Klengers http://www.klenger.net/dodge/general...calculator.xls

Aisen automatic, gears and tires are as follows:

35 inch tires with 3.73 at 1600rpm gives 71mph
35 inch tires with 4.10 at 1600 rpm gives 64mph

Since I want good economy on long highway drives I would probably go with the 4.10 rear end. I am not a speed demon and often I will be loaded with a camper so 64 mph is fast enough. Incedentally 100 rpm more (1700) gives me 69mph with the 4.10. So I can speed up if necessary. I have no idea what that extra 100 rpm does to economy.

 

Cummins says to gear for 2150-2250 at 65 mph for economy to power for the 5.9 and no lower than 1900. If you try to gear for 1600 at 65, any little puff of wind or hill will pull you off the torque hump. You also have to look at what your next gear will be putting you at if you have to downshift.
http://www.powerspec.cummins.com

 

It would be best to find a truck geared with 3.73 and another with 4.10 that are similar and ready to be hooked up to a trailer, if it sounds like it is lugging at 1600 with 4.10s then speed up to 1700. I'm thinking the best economy with the 6.7 will be around 1700-1900 while towing, but that is just me.

 

Duallydog

If 1700 - 1900 is the sweet spot - that is 69 - 77 mph with the 4.10 rear. So that means no tires greater than 35 inch could be run unless you drive crazy fast. Ok, I guess lots of folks go 80mph on the highway but that seems a bit fast for 37 inch mud tires.

That also tells me, anything over a stock tire and you can forget the 3.73 rear.

 

That's just a guesstimate, my word is not written in stone that that is the sweet spot. On a 5.9 it's around 1900-2100 for towing, mine with 3.73s is about 2000 rpm towing, if I go to 2100 it drops, if I'm at 1900 I loose speed with hills. In theory if I had 4.10s on my truck it would be easier for it to hold at 1900 rpms since the engine is working less to maintain that (Mechanical leverage).
I usually don't run over 70 and the area I'm living in right now most of the roads I'm on are 60mph roads. Other than I really want 3.55s empty out on the highways, 4.10s or a G-56 with 3.73s (last gear is just shy of NV-5600 with 4.10s) would work good for me. Or if I was towing my LQ trailer all the time I would love 4.10s. To me the perfect set up would be 4.10s and an overdrive gear splitter. Of course I've never run anything but stock size tires. You could use something like 4.30s if you put 35s on.

 

Unless something has changed, the C&C with the Aisin is available only with 4.10 gears. If you are getting a regular 2500 or 3500 pickup with the 68rfe transmission, I'm unsure of the rear end options.

 

That would solve the problem, but it would snuff out the discussion.

 

According to cars.com you can spec the 6 speed auto on the 2500/3500 with 3.73 or 4.10. They do not list the chassis cab model and Dodge only shows the 4.10 on the chassis cab as EB290 said.