Wishful Thinking?

[denandannie]

We're now on a seven week RV excursion beginning in Glacier NP, and working our way down through Yellowstone, Grand Teton, Rocky Mtn and ending at Arches in Utah on the way back to Nor Calif.

On a long, flat stretch on I 82 to Spokane WA, I did an experiment. I reset the truck mileage computer and then set the mph to 57. I drove that way for 30 min and then checked the mileage. I was getting 11.7 mpg on a 2015 Duramax . I then reset the computer and adjusted the speed to 60 mph, drove another 30 minutes and then checked the mileage. It had now jumped up to 12.5 mpg's.

This means either absolutely nothing, or it means the diesel performs better at a higher speed.

Any thoughts out there?

[mjammersc]

It depends on where the RPMs are (within reason). My 15 duramax at 57 is right on the threshold of wanting to downshift because the RPMs are lugging the engine down but a few MPH faster if has a much more consistent speed and no shifting issues. I've never tried the experiment like you have but have noticed when I am on 2 lane roads and have to stay around 55, it doesn't seem to do as well

[BB_TX]

The natural thought is lower rpms would equal better mileage. But if the engine is straining somewhat at those rpms then the efficiency will go down. But also, even slight differences in terrain, wind, pavement quality, etc can affect mileage.

[denandannie]

We have the same truck except mine is the long bed.

[DQDick]

Our Dodge averages 11.5 at 62mph which seems to be it's sweet spot. It gets less going from OK City to Albuquerque and then much better when we come back. It's not really noticeable, but it's up hill going west and down hill going east.

[Tfthorp]

Wind makes a big difference too. I have gotten 15 mpg with a tail wind and 6 mpg with a headwind.

[psomers]

Also if during your test period your engine goes into regen it will greatly effect your mileage. I believe the 2015 regens I know my 2008 does and I can tell when it is doing it by monitoring MPG.

[TLightning]

I run 62, that gets the Allison comfortably in 6th gear...no lugging.

[PeteandJoan]

Originally Posted by psomers

Also if during your test period your engine goes into regen it will greatly effect your mileage. I believe the 2015 regens I know my 2008 does and I can tell when it is doing it by monitoring MPG.

I have a 2012 Ford diesel and my mileage drops a noticeable amount also. Sure don't like that.

[Rondo]

Each diesel has a "sweet spot" when it comes to the speed and mpg's. I've tried mine a numerous speeds and it seems my "sweet spot" is at 62 mph and I have been averaging 12.4 mpg. Below on speed lugs the motor down and higher RPM seems to eat the fuel more on our 2012 Chevy!