I have a 2005 Dodge Quad-Cab, 4X4, SLT, Big Horn pkg. Just replaced the stock tires with BFG 315/70/r17's. With the stock tires, at 65 MPH I was turning 1810 RPM's. With my new BFG's at 65 MPH I'm turning 1650 RPM's. Haven't had a chance to experiment yet. Is 1650 lugging the engine too much? Anyone else out there with the same tires who might be able to tell me what speed is working for you? Also, is the overhead MPG meter accurate? Will it be accurate when I get my spedometer recalibrated?

 

1810 sounds a little low, what rear gear ya got? might be close 'cause I think at 70 I am just under the 2000 mark with the 373.... havent checked speed accuracy w a gps but 10 miles of milemarkered road gets me 10 miles on the odom so I think I got it right., the "sweet spot" for mileage is supposed to be under 2000, anything after that and down goes the mpg

might be a good time to fill out a sig line so folks know what ya have and what you have done to it.

 

If you are using your truck as a daily driver and not towing, 1650 is fine...doesn't hurt a thing and actually will give you better economy. Towing you may like to run a little higher in the RPM range....like 2000 or so.. even tho these engines can take alot more revs than that ..the higher you rev the worse the economy.

 

At May Madness, this year, Mr. Dennis Hurst, the Chief Engineer for our Cummins engines ( also, referred to as "the Father of Our Cummins Engines") gave a seminar with questions & answers. One of the things he said was that the "sweet spot" was in the 1800-1900 rpm range.

However, some folks have mentioned that they get their best mileage in the 2100-2200 rpm range. These trucks all have their own personalities & quirks.

Two of my friends have Duramaxes. One an '03, the other an "04. They both drove back from a camping trip last weekend. One got 25.5 mpg, the other ('04) got 19.7 mpg. Both verified by hand calculations. The '03 just runs exceptionally well. It's often driven harder, also.

I've noticed, with my "03, that with my trailer, on flat land I can get 15 mpg if I stay around 55-57 mph. 14 mpg at 60-62 mph. 13 mpg at 65-70 mph.

Joe F.(Buffalo)

 

I have 315s on stock rims, 48re and 3.73 gear. The 315s make effective gear ratio 3.43. I have had the "Pinion Factor" reprogrammed for the correct tire rpm. It has been verified GPS mph and Odometer distance readings.

Crusing rpm/speeds.

1800rpm = 70mph
1900rpm = 74mph
2000rpm = 78mph.

Engine seems to love around 1900 cruising. Most time on Texas roads I have cruise set just below 2000rpm. This averages around 16.6-17mpg empty in a nuetral wind. More with a tail wind and less with a head/cross wind.

 

I think part of that was in reference to another thread where I said that I had better mileage at 2100 to 2000 at 55 rather than 1600 in overdrive. I agree fully that the sweet spot is 1800 to 1900 on my engine also. But due to gearing on individual trucks you have to work with what you have in gearing. I can run the 1800 to 1900 at 65 and get great mileage and 70 at about 1950. I would need more gears to get into 1900 at 55 which would be optimum.

In my case I am set to optimum in overdrive at freeway speeds. If I lived in a 55 state I would have to make some changes. From what I have found building torque for towing does not change the 1800 to 2000 sweet spot, just makes the truck stronger and easier to maintain that rpm with heavy loads.

If you really wanted to get the best in mileage, you would need to gear for the sweet spot in direct, no overdrive. Each gearchange costs fuel mileage. This would be why mine likes the 2100 at 55 instead of the 1600 in overdrive. A comment on the wind stuff, if the wind is blowing I get worse mileage, I don't care where it is blowing from. My best mileage is with dead still and between 70 and 80 degrees. My trailer is a virtual parachute though, others may react differently.

 

Our engines make peak torque starting at 1600 RPM, so I doubt you are lugging anything.

 

I actually e-mailed powermaster at cummins about lugging the engine. They replied and defined lugging as running the engine with a load below 1600 RPM, or running up a hill in a gear where the truck would not increase speed/rpm when floored...

For my truck, anyway, speed seems to make a big difference in highway MPG, where RPM doesn't seem to matter much... I think they are pretty efficient from 1600 to 2200 or so...

 

i found that at 60mph i run around 1700RPM with 265's and 3.73's and my overhead computer showed about 3mpg less than it did when i was going 65mph at around 1850

 

This is where gauges will pay off. If you can keep under 10lbs of boost 2,000 rpm's and under 800 deg EGT's you will theoretically get your best mileage.

But another thing to concider is that the 325 hp and up motors the 3rd injection event supposidly shuts off at 2k and above which will really help mileage.

With my truck at 2k I run 9 lbs cruising and 700 deg at 67 mph. I have been able to run over 700 miles on one tank while doing this

 

0

 

Oh really? That's interesting-I'd never heard that before. If that's so, I can see how that might cause a "kink" in the fuel economy curve as you passed 2k. I've never noticed anything in my fuel mileage at different speeds that would indicate something like that is happening, but I've never really directly compared just over and just under 2k mileages to see. How sure are you that the 3rd event is shut off at 2k? I can see that being a nice feature to get in an aftermarket box. I don't see why they couldn't do that pretty easily, especially with the boxes that have their circuitry inserted between the injection harness and the injectors, like the TST does.

 

From the "Rock Solid Rules" in the Cummins MPG guide: http://everytime.cummins.com/every/p...Whitepaper.pdf

It says: "Above 55 mph, each 1 mph increase in vehicle speed decreases fuel economy by 0.1 mpg."

Lot's of other good tips in here.

 

If you play with Cummins PowerSpec software, you will see it says 2150 for best combo of power and mileage where it says 2250 for power, both at 65mph. I would think this is for the truck being loaded. It does give the same info if you put in 40,000 lbs for the weight. You can play with the specs for different engines and powertrain combos.

 

My understanding is that diesel engines with common rail injection and a catalytic converter generally use a late injection event periodically, and that this late injection does not produce much for power but is intended to heat the catalyst to burn off carbon and other contamination. By monitoring conditions, the engine management computer determines how often and when this needs to happen to keep the catalytic converter working and unclogged. Apparently this is largely responsible for a slight reduction in fuel efficiency in engines with a catalytic converter verses previous versions of the same engine not so equipped.

If this is correct, then the explanation mattymac posted seems to make sense…