Fuel Economy Sweet Spot

[CToney]

I've been told by a few owners that our trucks sweet spot is ~120km/h (75 mph). I usually drive 110 km/h most everywhere. Most of the time my mileage goes between 10-11 L/100kms. Recently I decided to try driving faster. Instead of revving 1820 rpm I was 1960 rpm or so. Mileage seemed to be 9-10 L/100 kms. I'm from the school where speed hurts mpg. Is 1960 rpm the sweet spot. What do guys with 3.92 gears rev doing 68 mph?

 

[Missouri Mule]

I forgot the term but it I is not sweet spot.
As I recall by Cummins/Dodge or our editor it the formula is in one of the the mags. As I recall My Mule swt spot is at 53 mph in 5th gear. That is the speed Mule should get the best mileage. As I recall that should be at 1600 rpm when Mule reaches max torque of 440.
Since (As I recall) the commons rails can advance timing your peak torque what ever rpm is should straight line to the rpm where the engine reaches peak horse power. (Usually both start to drop after that.

 

[HPSimpson]

So today driving on highway with 3.92 @ 1950 rpm 68 mph; as normally I usually have two axle boat trailer towing .

 

[CToney]

Thanks for the reply. By the looks of it to me if we had a 3.73 gear set it would be at the perfect rpm for the speed I drive. From the Ecodiesel FB groups and everyone running 35" tires they should have 3.92 gears for better mpg. I don't plan on towing much at all. I'll try to find out what I can to maximize the life and fuel efficiency out of my ED.

 

[The patriot]

Peak torque?
BSFC?

 

[CToney]

Peak torque?
BSFC?

Yup BSFC was the term I was looking for.

Cooler temps have hurt my mileage as of late. Longer warm up times to get to operating
Temp hurt mileage. We live 15 miles from town. It takes at least 7 to get up to temp. I'm going to put an oil pan heater on the truck to help speed this up some.

 

[The patriot]

Good idea, but I highly doubt it will help MPG's.
Winter blends and cold intake temp is hurting mileage.

 

[slowmover]

Best mpg wont ever be over 60-mph.

Anything over 65 won’t pay on a 70-mph Road.

Aero resistance is what matters. Not gearing or the rest.

EDIT: Now I see who I’m talking to, ha!

 

[HPSimpson]

Don't you have engine block heater , try that first plug in at night, use clock to turn on the heater couple hours earlier, that will make big difference.

 

[slowmover]

Yup BSFC was the term I was looking for.

Cooler temps have hurt my mileage as of late. Longer warm up times to get to operating
Temp hurt mileage. We live 15 miles from town. It takes at least 7 to get up to temp. I'm going to put an oil pan heater on the truck to help speed this up some.

And get the MOPAR winter front. $150. 41F or lower. Use block heater.

I’d add oil pan heat pad.

Coolant temp doesn’t mean much. These fancy turbos ratchet that up. It’s the oil temp that matters for mpg. In summer, in Texas, that takes 45-miles in CTD or Kenworth. Loaded.

Tires take 1.5-hrs to equalize.

Greases, etc are in between.

An insulated garage is what will make that thing last longest at lowest cost. Anywhere in world. Pays for itself by keeping one from buying new vehicles prematurely.

The garage is the equalizer.

If not that then a Webasto. Route exhaust near oil pan if feasible. Avoid hot spot (150F localized starts to be a problem for oil chemistry; cooking).

You know it’s about the annual average.

Usual 4WD CTD owner at 15-mpg average has spent on fuel and maintenance plus repairs and upgrades TWICE what I have before 250k and 21-mpg with 2WD.

Premature wear above 65-mph or so will run your cpm of ownership/operation thru roof.

The current CTDs are way past $1.00/mile.

Do the numbers on projected fuel over expected lifetime. It’s an eye opener.

 

[CToney]

Truck is in heated garage. I do use block heater. If plugged in for 1 hour ECT is ~80F. My CTD would be 90-100F. My coolant temps with winter front on are 212-215 with it 30F and my oil temps are 228-230. Kinda warm if you ask me. My CTS 2 doesn't monitor IAT. It hasn't been that cold here recently. It's strange to have boost and engine load drop with a higher speed...

 

[The patriot]

Mostly to do with class 8 trucks, but still some good info:
https://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf

 

[CToney]

Good article. It reminded me of a few things. Tires, my tires are near new 4000 miles and my engine is near new as well. After breakin mileage should improve. Tires are lug type as I have an Outdoorsman. 265/70R17 when they wear out I'll look at getting 245/75R17 tires. Hopefully they weigh a couple pounds less and should help mpg.

I'll keep playing with the truck and get it sorted out I'm sure. I wish Edge would come out with more PIDs for the 17s. It sucks to not have soot %, rail pressure, IAT, more than one EGT.

 

[slowmover]

Good article. It reminded me of a few things. Tires, my tires are near new 4000 miles and my engine is near new as well. After breakin mileage should improve. Tires are lug type as I have an Outdoorsman. 265/70R17 when they wear out I'll look at getting 245/75R17 tires. Hopefully they weigh a couple pounds less and should help mpg.

I'll keep playing with the truck and get it sorted out I'm sure. I wish Edge would come out with more PIDs for the 17s. It sucks to not have soot %, rail pressure, IAT, more than one EGT.

VernDiesel is a commercial operator of your truck with about 300k on truck. Some mods. Ex-truck driver now delivering Airstream TTs. Has some backhauls that makes it pay.

Supposed to be a mod on a dedicated forum. I’d search out what he’s done and ask some questions. His posts on www.airforums.com show me a guy who gets the details.

Oil temps above 220F are what enable best economy. Cool coolant and hot engine oil. Ability to control oil temp is what marks modern engines. Thus, adjusting WF airflow flaps is to avoid overshoot. Ambient air temp first, then engine load.

Percent Engine Load is more useful than other gauges mentioned. Avoid 80%-plus and the rest is easy enough.

The old rule was easy enough. 666. Don’t exceed 60-mph, 600F EGT and 6-psi manifold pressure.

Work with what you have. The correlation between average mph and average mpg when you’ve set boundaries on how you operate the truck.

75-mph isn’t the thing any more than extended idling. Etc.

 

[Michaelsloft]

Mostly to do with class 8 trucks, but still some good info:
https://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf

Cummins has some good information on the website. Thanks for posting this link for fuel economy. I found the road surface effect on fuel economy on page 26 very interesting and more rural roads/highways are being resurfaced with Chip and Seal Blacktop to save funds! Not to mention the increased tire wear and overall vehicle wear and tear.

Road Surface
Even road surface has a documented effect on fuel economy.
Using new concrete as the standard baseline, worn or polished concrete is even better. All
other road surfaces are worse, some substantially.
Table 7: Tire Rolling Resistance Pavement Type and Condition
versus Relative Rolling Resistance
Relative Rolling
Road Surface Resistance %
Concrete polished (best mpg) –12%
new baseline
Asphalt with finish coat 1%
medium coarse finish 4%
coarse aggregate 8%
Chip and Seal Blacktop (worst mpg) 33%
Road roughness can increase rolling resistance up to 20% due to energy dissipation in the
tires and suspension (10% loss of mpg)

 

[Killgore]

Good thread, although I have to keep reminding myself that your discussion centers around running the Ecodiesel efficiently. That being said, the same principles apply to the efficient operation of the CTD. When unloaded, I try to not exceed 8-10 PSI boost and 600*F while turning 2000-2100 RPM's which is around 64-65 MPH (G-56 with vintage AD gearing). When pulling my 18,000 lb. "home", all bets are off, especially when running with the flow of traffic and trying to avoid hindering others. At that point my MPG slip to around 8-9, and I am more concerned with being able to stop than go, LOL.

- Ed

 

[JR]

Heated garages are the worst thing you can do to a vehicle if it's driven on treated roads. Allows the vehicle to warm up and the chemicals literally eat your vehicle away while you sleep. Brooming snow and ice off your vehicle every morning sucks but as long as the temps are cold enough, and I've heard different temps based on which chemicals are used, the oxidation effect on metal is relatively neutral.

 

[CToney]

I have a few miles of gravel roads to drive on on the way to anywhere. Rolling resistance goes up a lot on them compared to pavement. Boost and engine load go up a lot and milage suffers.

I understand that higher temps will speed up corrosion. They don't use a lot of salt in our area and we don't get much snow.

Hopefully Edge comes out with updated PIDs for the CTS2 I'd like to monitor more parameters. By the sounds of the 2017 ECM it's a tougher nut to crack. PPEI doesn't tune them yet. I'm not sure on Celtic or the others.

I'm wondering if I should put gorilla tape between my Leer cap and cab. There's a large gap near brake light.

 

[TCDiesel]

Heated garages are the worst thing you can do to a vehicle if it's driven on treated roads. Allows the vehicle to warm up and the chemicals literally eat your vehicle away while you sleep. Brooming snow and ice off your vehicle every morning sucks but as long as the temps are cold enough, and I've heard different temps based on which chemicals are used, the oxidation effect on metal is relatively neutral.

JR.. not all Garages are equal, My garage Has 3 different types of Heating and two Cooling types of cooling.Mini. Sealing and electric, Forced, it is always around 30% humidity, It is heated in the Winter and cool in the Summer, The System is designed to remove up to 30 Gallons of water Per-Day..When the vehicles enter and park the Systems literally removes the H2O of the surface it will remove the standing water in 2-4 Hrs max. if You don't spent the $$$$$ in control environment around here with just the standard Heating the Humidity will skyrocket and Your vehicle will rot away before your eyes. Like JR mentioned.

 

[slowmover]

Good thread, although I have to keep reminding myself that your discussion centers around running the Ecodiesel efficiently. That being said, the same principles apply to the efficient operation of the CTD. When unloaded, I try to not exceed 8-10 PSI boost and 600*F while turning 2000-2100 RPM's which is around 64-65 MPH (G-56 with vintage AD gearing). When pulling my 18,000 lb. "home", all bets are off, especially when running with the flow of traffic and trying to avoid hindering others. At that point my MPG slip to around 8-9, and I am more concerned with being able to stop than go, LOL.

- Ed

Running “with” traffic means incorrect vehicle spacing. Terrible habit. Let the idiot packs get out out ahead. And don’t sweat it.

55-65/mph is the cruise range. And far from other vehicles. One uses ones mirrors to predict the future, as in: the future us behind me and coming on. Manage the flow by getting others around as quick as possible.

I spend my days cancelling cruise to do just this. I want no one near this smoothbore tanker if I can help it.

And you aren’t a hindrance to traffic. Only to the stupid people. Risk reduction is vehicle spacing. Over 700’ preferably. No one beside me or ahead or behind me.

The Third Worlders literally incapable of abstract reasoning can’t make deductions based on mirror images. Why they come down ramp at 50-mph. They also crowd the bumper ahead as they can’t conceive of a wreck at 75 being worse than one at 35. DONT LET THEM PUSH YOU.

FE is finesse. Within those statistically valid guidelines. Your chosen speed is of little consequence per that. 60-mph is the aero wall. FE drops dramatically past that.

Frankly, if one is gaining on traffic (nearing a metro area) then back off again. The lower the set speed, the better. Just ease along all day.

If serious about FE, then the daily count of acceleration and deceleration events is central. As are lane changes not associated with exits to left. Obviously, the fewer, the better. This is to establishing new habits. The insights.

4WD trucks big fault is bad steering. Any slop at all is unacceptable. Tires should be to load, not max. Anti-roll Bars (matched) are a help, as would be the Super Steer Rear Panhard Rod. Shocks should be AT LEAST entry level Koni or Bilstein.

The control for MPG towing is 40% penalty. Truck at same weight less TW, and same speed over same road. Set up a loop from home. Fill before and after. TT axle alignment is always suspect, as is bearing preset. Brake set.

Truck problems are also alignment & caliper drag. Check also no leaks in CAC.

I bought my rig (truck & trailer) with high FE as important. 35’ TT. 18k GCWR. And 17-mpg proves out as exact 40%. (Average is 15).

I run 59-mph wherever I go. On trips under 300-miles there is no significant time difference. And all other tasks at the wheel are easier. Far wider cone of peripheral vision.

Plan the day in advance. All decisions about stops scanned by satellite pic the night before. Just execute each leg. Fresh and relaxed after a break every two hours or 100-miles.

It’s new habits. New way of looking at it is the reason I responded to your post. I’ve been at it a lifetime. Most never kick the 16 year old or car commuter out of the drivers seat. You’ll fight with yourself. Everyone does. Have to do it.

You can’t brake, steer or maneuver the rig with anyone around you. Screwed, blued & tattooed.