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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.
In my view, diesel engines and fuel efficiency follow a simple formula: Run the engine as close to its torque peak, and you'll realize the best fuel efficiency. We have a 2005 Dodge Ram 3500 Cummins 5.9L, stone stock engine, no "chip" or exhaust modifications, the truck is just as it came from the factory (purchased new in fall of 2004). We have a friend with a 2004 model, similar, and we've compared fuel mileage for years now. I've gotten as much as 25 mpg on an unloaded trip from east of Reno, Nevada to Portland, Oregon. I've pulled a loaded car hauling trailer (Jeep XJ Cherokee on board) to Moab, Utah and managed 17-18 mpg at interstate speeds. Even after "lifting" the truck and adding oversized tires plus enough accessories to push the curb weight above 9000 pounds with fuel on board, I've coaxed 22-24 mpg out of the truck on flat interstate runs. So, how is this possible? Very simple. I'm an ex-heavy equipment operator and know diesel engines. These engines have a quick torque rise, more so the Cummins among the light truck applications. This means that torque comes on quickly, peaks as horsepower builds, and the overall rpm range of the engine is way less than a gasoline engine. Note: My lesson for all this was running heavy equipment "in the day", primarily with 1693 Cat engines: At 893 cubic inches, these inline six monsters would reach 1090 lb/ft peak torque by 1000 rpm—that's just off-idle! Our friend seldom achieves more than 18 mpg from his Dodge Ram 2500. He also has the NV5600 six-speed manual transmission, and I have the "inefficient" 48RE four-speed automatic. What's wrong with this picture? The engine operating rpm and our driving technique differences. A few years back, I asked my friend what rpm he uses for shift points. His reply was 2500 rpm. The 5.9L Cummins H.O. inline six peaks its torque at 1600 rpm and redlines at 3400 rpm. In my experience, optimal fuel efficiency on the highway with this engine has been in the 1600-1900 rpm range, the best mileage achieved around 1600 rpm when not under load. Overall, for fuel mileage, the shifts points for this engine should be 1400-1600 when unloaded, 1600 if possible when loaded. There are times, of course, when the 1600-1900 rpm range is necessary to keep a load moving, and even higher rpm may be necessary for acceleration and climbing grades. When I modified the '05 truck with the lift kit and aftermarket accessories, adding a good deal of weight in the process, the original axle gear (3.73 with OEM tires) was no longer viable. While I believed the "overdriving effect" of oversized tires might benefit mileage, the added load and taller gearing effect actually decreased mileage dramatically—especially trailer pulling. In selecting axle gear sets to compensate, the new 35" diameter tires required 4.10:1 gears for a direct speedometer correction. I considered the new, unladen weight of the truck and our plans to pull trailers. My choice was to go even lower (numerically higher) on the gearing. The AAM 11.5" and 9.25" axles do not offer a ratio between 4.10 and 4.56:1, so I went with 4.56:1. This raises the engine rpm at a given speed when compared to the OEM gearing with the original tire diameter. For trailering and the new vehicle weight, I thought the trade-off worthwhile. Note: Cummins actually recommends 2100-2400 rpm for peak efficiency in commercial use of the ISB diesel engine. They would like to see 2100 rpm at 65 mph and no operating below 1900 rpm under load at highway cruise speeds. In stock form, 1900 rpm netted approximately 69 mph. Switching to 4.56:1 gearing, 2000 rpm (with overdrive at 0.69:1) nets close to 65 mph. True to my expectations, my peak fuel efficiency is now at 65 mph or lower, a calculated change. If I hold the truck to 55-65 mph, the unloaded peak mileage is 23-24 mpg. For trailer towing purposes, California caps at 55, Nevada allows for 75 at best, with 65 being plenty of speed for trailer pulling—if you care about mileage...This means watching rpm during upshifts as well. I'm very pleased with this truck's fuel efficiency. Perhaps a chip could improve this further, although my belief is that driving technique holds far more sway over fuel efficiency than any other factor. If you have a 5.9L Cummins and would like to experience better fuel efficiency, watch your tachometer. If I creep over 1950 rpm, the price will be a linear increase in fuel consumption. By 2100-2200 rpm, fuel efficiency, reflecting load as well, begins to drop like a rock... Moses