15.6 mpg highway?

[seahawkdodge]

This is the third Cummins truck I have had. 2016 crew cab, Laramie 4x4. 68RFE transmission, 3.42 gears. It has 34" Michelin AT2 tires, just one size up from stock. Just about 54k miles.
My issue is that I am getting a solid 15-16mpg highway. Granted, the speed limit is 75 for most of my drive, so I typically set the cruise at 80.

I had a 2017 with 3.73s and stock Trashforce tires that would run 22mpg at 75.

Any ideas? Besides the mileage, it seems to have no issues.

 

[Ozymandias]

The gears make the difference, your engine has to work way harder now then the old one with the 3.73.
3.42 is to low from my point of view.

 

[bcbender]

Also 4x4.. FWIW, my 20 gets about that 15-17 hwy at 75~80 but if I slow down to 65, she gets almost 20. Are your numbers from the EVIC or hand calculated?

 

[Cummins12V98]

The gears make the difference, your engine has to work way harder now then the old one with the 3.73.
3.42 is to low from my point of view.

At 75 mph???

 

[Cummins12V98]

Op you need to calculate percentage the tires are off.

 

[Ltngdrvr]

This is the third Cummins truck I have had. 2016 crew cab, Laramie 4x4. 68RFE transmission, 3.42 gears. It has 34" Michelin AT2 tires, just one size up from stock. Just about 54k miles.
My issue is that I am getting a solid 15-16mpg highway. Granted, the speed limit is 75 for most of my drive, so I typically set the cruise at 80.

I had a 2017 with 3.73s and stock Trashforce tires that would run 22mpg at 75.

Any ideas? Besides the mileage, it seems to have no issues.

What 2017 truck had 3.73's???
Not factory.

And, a 4x4 truck running 80 is doing well to get 15-16.
The larger tires don't help any either.

And, are you going by the dash mpg readout or calculating it???

 

[seahawkdodge]

It is all hand calculated. The lie-o-meter is relatively close though.
GPS has the speedometer within 1mph.
80mph is like 1700rpm.

My 98 12valve ran 2100rpm at 73mph.

I would have to go back and see if I still have the sticker for the 2017, but I was pretty sure it was not the 3.42 gears. And not the 4.10. It wouldn't be the first time I have been wrong though.

 

[AH64ID]

Was the 2017 a DRW? That’s the only way to get anything but 3.42’s.

22 at 75 seems abnormal, not 15.6.

Have you corrected the odo for the larger tires?

 

[BigPapa]

80mph at 1700 IS abnormal, no?

 

[AH64ID]

80mph at 1700 IS abnormal, no?

Depends on what size tires he has. 1700 rpms is 76 mph on the stock tires.With an accurate speedo it would take a tire with ~595 rev/mile to be doing 80 at 1700. Stock is 626 rev/mile (275/70R18).

I didn’t see a 1 size up offering from Michelin in the AT2, so the op should post his actual size.

 

[seahawkdodge]

They are 275/65/20, if I recall. 34.1" according to Michelin.

AH64, I probably should have just kept the 2017. It would have been the smart thing to do.

 

[AH64ID]

610 rev/mile to the stock 622, or 2% larger. They roll at 33.1”.

80 mph would be 1750 rpms with those tires.

Was the 2017 a DRW?

 

[seahawkdodge]

No, the 2017 was a SRW, crew cab short box. Apparently it was a 3.42, I just had 3.73 stuck in my head for whatever reason.

A dually getting 22mpg...now that would be something.

 

[AH64ID]

A dually getting 22mpg...now that would be something.

As would a SRW at 75….

 

[seahawkdodge]

As would a SRW at 75….

I took it on multiple long trips, 21-22mpg was the norm with it. Hand calculated.
My 98 12 valve would do 20mpg, as long as I stayed under 70.

 

[kthaxton]

It wouldn't be the first time I have been wrong though.

Can vouch for that.

My 2015 gets the same (15-16) mixed driving. It gets noticeably worse when I exceed 70-ish mph.

 

[seahawkdodge]

Ouch...

Can vouch for that.

My 2015 gets the same (15-16) mixed driving. It gets noticeably worse when I exceed 70-ish mph.

That

 

[Topzide]

I can say that with summer fuel, I can get 16 - 17 mixed use driving and darn close to 20 on the highway staying under 75MPH! Dash will show 19 mixed and 22 on highway! Winter fuel is about 2 mpg less. DRW and 4.10! All hand calculated.

 

[slowmover]

This is the third Cummins truck I have had. 2016 crew cab, Laramie 4x4. 68RFE transmission, 3.42 gears. It has 34" Michelin AT2 tires, just one size up from stock. Just about 54k miles.
My issue is that I am getting a solid 15-16mpg highway. Granted, the speed limit is 75 for most of my drive, so I typically set the cruise at 80.

I had a 2017 with 3.73s and stock Trashforce tires that would run 22mpg at 75.

Any ideas? Besides the mileage, it seems to have no issues.

Congratulations on achieving while solo what mine averages towing my 35’ TT.

But you’ll get “there” 40-minutes ahead of me on a 300-mile day.

Diesel pickups need a plan. Excessive speed ain’t part of it. Can’t slow and sure can’t maneuver.

The Average MPG (all miles & all gallons) has to be nearly 18-MPG to make a 350k mile engine life estimate given that Average MPH can be kept above 30-MPH. “Steady State” matters, not mpg at some rice burner road speed.

Fewest cold starts, no idling, makes every stop light, etc. = MPG. (And more).

Make a plan plays to strengths: ability to do work greatest number of miles with highest reliability at lowest cost. Cut out “stupid”.

The only MPG figure that matters is the Average. All miles + all gallons.

Separating Highway from City MPG isn’t relevant except to note how low one can keep the percentage spread between them. Can’t get it down to 10%? Why not? (It’s only a question of steady state). Highest highway MPG is always under 60-MPH. Get that baseline number and apply to city mpg to work the percent spread downwards.

The operator is the problem which produces results given a plan. One cares about cost, life, reliability and can still work hard with high miles, . . or he doesn’t. (Actions prove it).

Tire & brake life are the other indicators past MPG that a plan is in effect and successful. Or not.

At $4/gal price of diesel, I worked my city average percent down to 11%. Made a plan. The savings was such it funded 5,000-miles of towing my trailer for the same annual fuel budget.

So you blew past me on the highway at the same MPG. Just remember that my vacation fuel was free.

In the past twenty some years of online forums this “problem” of vehicle blamed for driver inability to adapt hasn’t ever changed.

.

 

[seahawkdodge]

Not really trying to be a smartass here, but 70mph gets me run over on the interstate. I live in Texas, and the speed limit is 60 close to work, and quickly ranges up to 75. Me doing 80 in a 75 gets a lot of folks huffy as it is.

It seems the consensus is that there's nothing mechanically wrong with my truck. At that speed, it just isn't efficient. Maybe a tune might help a little bit, maybe some highway tires. But at 80mph, 16mpg is the reality.

Thankfully I don't drive this thing every day, 71 miles each way.
That's what my Volkswagen TDI is for, 38 mpg or more.