My guess would be that you didn't fill the tank to the same level and that caused the difference you probably got 14 one way and 22 the other way.
Since I keep track of every tank I do see the times when I fill the tank to the max at one station and don't get it full at the next one because of slope of the ground or the pump shuts off faster.... That's why I keep asking for an AVERAGE because just a gallon on a 300mi tank is what? 300mi 17gal = 17.6 300mi 16gal = 18.75mpg, 300mi 15gal = 20mpg, 300mi 14gal = 21.4mpg
I would venture to say a lot of those partially filled tanks are reported at 20+mpg when if they AVERAGEd the next tank with it was only 17mpg.

 

It is tough to get 20 mpg out of these trucks...actually, any pickup. For me, the key factors have been:
- speed. the truck does great at 40 mph, great at 55, good at 65 and drops considerably at 75. The best highway speeds for me are around 55 to 65 which is where I hit 20mpg. At 75 I cannot maintain it ...it drops. The calculation is not the overhead - full fuel fill up...drive it, return to the same station, refill, divide miles driven by the number of gallons = mpg. You cant trust the overhead. Sometimes it is close...it is always slightly optimistic. I have averaged over several trips and my typical highway mileage is 19.
- stock tire size. I run the Michelins in the better weather months. The truck does better in the winter with a more aggressive tread. But that doesnt help mileage.
- bone stock with one exception, a cold air intake after market. It might add a little but probably wont return given the cost.
- change fuel filters about every 10000 miles. Not sure if it helps but I run a small percentage of commercial biodiesel so I wanted to be safe.
- avoid drviing it in town as much as possible. The mileage really is poor in town.
- if you put the slide in camper in the back...it is best to not calculate mpg. It creates the large toaster aerodynamics effect.



2007 QC 4x4, mineral gray, auto.

 

Driving out to WV and down to SC, with the XZT back to a milage program, the overhead reads 22, and calculated averages about 21.2. That's driving 69 mostly, 74 through parts of WV and NC, and 55 on the back highways of SC. I also set the transmission switch to "tow mode".

 

After 16K, I get about what my 5.9 delivered. Maybe, 1 mpg less worst case.

The difference is not enough to give up the 6.7's performance advantages. For a working truck the 6.7 auto is better rig all the way round' than my 06' 5.9.

 

I get about 16 in town driving and 19 to 20 on the road. 18 to 19 at 80 on interstate. Phoenix to Kansas City with empty car hauler and got 16 mpg, coming back with a 34 Ford on it and got 14. About the same with a Honda Accord from Phoenix to Ft. Rucker AL and back empty. Pull a two horse trailer and get about 14 to 15 with two horses and 17 mph empty. From Phoenix to Branson MO., to West Point, NY. to Washington DC, to Nashville and Dallas and home with 30 ft toy hauler, got 11 MPG. Stock 2006 Dodge truck with stock tires, 343 4X4.

 

By the looks of the replies most of these numbers are from 5.9 truck and not 6.7's!
I have a 2007 quad cab, Cab n chassis with a 9' hauler bed, auto, 4.10 gears and was only getting 11 to 13 loaded or unloaded till I hit 20-k miles, now unloaded driving under 70 I can get right at 15 and loaded with a 20-k horse trailer I get just under 12 and these numbers are figured by hand and overhead computer. They both stay with in a mile or 2 of each other.

 

My mileage has improved up to now. With winter blended fuel I am still seeing 19 on the hwy running at 68-72 mph. If I hit the magic number of 75 I see it drop to 17.
Loaded I can get as high as 13.8 and an average of 12.2 now. Thats an average of all my trips with some terrible ones in there prior to my sugery on the egr etc. On an optimum day I can get 21 to 22.8 but thats with a speed in the 60-62 mph range empty and at 55-60 I can net high 14 mpg...I learned this in a controlled environment...it was a long weekend and heavy traffic...LOL
It was nice to see but it felt like I could get out and walk. My average after the surgery is higher then my 07 5.9 3500 quad cab auto dually 4x4. Not by much but it is better.

Hope this helps.

Scotty
I should have mentioned...the truck I run now is a 6.7 Mega Cab stretched, auto 3500 4x4 dually.

 

I guess the 5.9 guys responding must think they are in 3rd gen 5.9 section.

I am watching this thread for 6.7 mileage findings not the old 5.9's. I am thinking about swapping the gasser back out for a diesel. This time it would be a 2500. No dually again.

 

I Live on flat ground in Florida. I drive 40 miles round trip to work. 2/3 of that is limit access highway at 55mph and the other 1/3 is 50mph. This gives me 23.5mpg over several tank fulls. The overhead is showing 25.8 on a regular basis. Interstate doing 70 or 75 I get 20mpg. I have on occasion seen 19 but never below that. Towing 10,000 pound box trailer with a V nose, (I went across the scales and lacked 60 pounds of being 10,000) and running 60mph to the foot hills of North Carolina, I got 14.5 I drive about 750 miles before I fill up. I love my truck

 

The guy that runs the local rental store has a 07 3500 6.7 and a Sterling
He said he drive his truck hard and is lucky to see 15mpg that's mixed city/hwy. He's had to have his soot filter cleaned and said his buddy that pulls a trailer all the time has had his cleaned also.

 

Over Thanksgiving weekend, I drove from Boise to Idaho Falls ( 286 miles per the gps ) and NOT via the interstate so it was mixed 2-lane roads with an average speed of around 65 and I got 16.9 hand calc'd - my overhead read 20.5.

 

2008 - 6.7 --68RE
7000ft elevation, Butte Mt.
10 mph headwind, 10 degrees
17,580 miles
16.8mpg-17.6 72-75 mph
towing 21ft trailer (3 horse and supplies) ~9000lbs ---12.5-14.6 mpg
nearly the identical mileage for the same tasks as my 2002 --2500 dodge
The overhead calculator is a complete waste. Why did they change it from the one that worked so well?

 

About 16 with winter fuel right now. I don't baby it either.

 

I'm sorry!

I thought it was Ram Diesels all.

To get my 19.3179 overall average I spend a fair amount of time at 60 and RARELY exceed 65.

I have never heard or read anything that would make me want a 6.7!!!

 

i get 16.3 around the farm and highway 22-24.2 at 64mph 70 mph goes down to 21