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Steve Atkins

High Fuel Efficiency and Mileage with 5.9L Cummins

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I recently purchased a 2001 Dodge Ram 2500 to put put a test engine in it. Before the engine swap I wanted to get some base line mileage from the current stock engine. From what I had read and had been told I needed to get my RPMs down in the engines torque range. I am not hauling trailers so the gearing I picked was strictly for millage. First I installing a tall narrow tire 295/70/17. My truck had  the stock 4.11 gearing.  I was getting 18 MPG HWY at 68 MPH at 2125 RPM. Now that I had some numbers in hand so a professional could do the math. My target was 1800 RPM at 70 MPH. Tom at Reno Driveline and Gear came up with 3.54 gears.  I wanted to go with a higher gear but my axels where my limitation. 

My first run at 1750 RPM took me buy surprise. I got 27.24 MPG (65 MPH). I could hardly believe the results! I then dropped my RPM to 1625 and I got 27.85 MPG (61MPH). 1875 RPM got me 23.2 MPG. (71 MPH).  1775 RPM got me 23.4MPG (68MPH) still not bad.  I actually could not have been happier with my results. 

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That's great , Steve...Well worth doing the math.  For fuel efficiency, it's valuable to drop your engine closer to 1600 rpm like you discovered.  My best fuel mileage with stock tune was always 1600-1700 rpm with our '05 Ram H.O. 5.9L Cummins.  1600-1750 rpm works nicely! 

Part of this is the aerodynamics (lack of) and vehicle dynamics.  For these trucks, speed increases the resistance and load.  Speed is a significant factor.  When I hold to 65 mph running empty, I can coax 21-22 mpg, sometimes even 23 mpg with a consistent tail wind.  My truck's substantially oversized tires, chassis lift, 16,000# capacity front winch and HD bumper, auxiliary fuel tank, bed cap, and all of the other add-on weight (total in excess of 1000 pounds with a full tank of auxiliary fuel!) would be the difference between our two trucks.

I use cruise control on I-80, 67 mph tops.  When I exceed 1900 rpm, the price in fuel use is clear:  2100 rpm is my torque peak with the Hypertech/MaxEnergy software tuning.  I'm 16-17 mpg at that engine and road speed (72 mph).

As a point of interest, watch for my pending article on the BD overdrive lockout for the 2005 48RE (the only year without a manual overdrive lockout in stock form).  If you have the automatic (47RE), you should have the factory O.D. lockout for pulling grades.  Slow down and use it!  This will save the transmission by holding in 3rd gear on grades and not taxing the overdrive unit. 

I now use O.D. strictly at cruise and under lighter loads like when towing on flat ground.  Use your intuition.  I'm planning for 50-55 mph when towing 6% grades.  (This is 2100-2300 rpm in my case, peak torque is 2100 in present tune.  Your 5.9L Cummins diesel engine's peak torque is 1400 rpm if non-H.O. or 1600 rpm if H.O. version.)  My transmission is intact and fully functional at 162,000 miles.

Interesting how quickly rpm impacts fuel efficiency...Thanks for sharing this dramatic illustration of engine rpm versus fuel efficiency...You're doing well here!  Keep us posted...

Moses

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Thank you for the input Moses. I failed to mention that I have the manual 6 speed, NV5600. The only other modifications done to the truck are a front load leveling kit, 5" exhaust and 55 gallon fuel tank. The tests were of course as controlled as I could make them so my real life driving will vary.  I still have easy second gear starts and now do all my driving between 1400 to 1800 RPM. I will post more mileage specs as they develop. 

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Sounds good, Steve...The NV5600 offers way more control for fuel efficiency and performance.  Upshifts without a load should be around 1400 rpm.  The 2nd gear start capability means you did not go overboard with the taller 3.54 gearing...Good job!

Moses

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