Can the 6.6 gas HD hit 20mpg hwy? Got close.

[Pryme]

I’m impressed with this trucks mileage. Drove it 250 miles this morning and knocked down a best of 19mpg. Of course this is not towing anything and no real load to speak of. 
 

calm sunny day on the fwy. Avg speed 68/72. 68 in the 65’s and cruise set at 72 in the 70 zones. 
 

windows up and ac on. 
 

here are some pics off the scan gauge. The dash was consistently .5-.6 under the scan gauge. 
 

 

[Pryme]

[Pryme]

[Chuck FB]

I did not think these trucks would be able to achieve that type of fuel mileage at that speed. Is that section of road quite flat and at what elevation is that at ?. Also do you know if the fuel your using is non ethanol or if it has the typical 10% or whatever it is that they throw in. 

 

What is your trucks 25 mile "best" reading, while that is definitely a cherry picking number I'd be curious what it shows as mine lately for whatever reason of I would guess a tail wind and maybe driving slower jumped up to a 21.2 reading in the 25 mile. I'd be blown away if I was able to reproduce that on an actual drive. Having said that I have yet to be on a drive longer than a 50 mile drive from point A to B and with a couple of stop signs and grades to climb as well so not a good test for best fuel mileage. Also and expect you have noticed this as well, right out of the gate from a cold truck the fuel mileage is in quite a deficit and as the miles accumulate the mileage keeps increasing slowly as a cold driveline and a cold engine even in summer time temps takes a while to warm up and lower the initial friction/drag. It would be interesting for you to drive the same route with the same weather at 60 mph and see the difference, you may even break past the 20 mark !  

[Pryme]

35 minutes ago, Chuck FB said:

I did not think these trucks would be able to achieve that type of fuel mileage at that speed. Is that section of road quite flat and at what elevation is that at ?. Also do you know if the fuel your using is non ethanol or if it has the typical 10% or whatever it is that they throw in. 

 

What is your trucks 25 mile "best" reading, while that is definitely a cherry picking number I'd be curious what it shows as mine lately for whatever reason of I would guess a tail wind and maybe driving slower jumped up to a 21.2 reading in the 25 mile. I'd be blown away if I was able to reproduce that on an actual drive. Having said that I have yet to be on a drive longer than a 50 mile drive from point A to B and with a couple of stop signs and grades to climb as well so not a good test for best fuel mileage. Also and expect you have noticed this as well, right out of the gate from a cold truck the fuel mileage is in quite a deficit and as the miles accumulate the mileage keeps increasing slowly as a cold driveline and a cold engine even in summer time temps takes a while to warm up and lower the initial friction/drag. It would be interesting for you to drive the same route with the same weather at 60 mph and see the difference, you may even break past the 20 mark !  

It’s fairly flat but not Nebraska flat. There are some inclines and decline. Probably 800ft elevation. It’s basically across the middle of Wisconsin. 87oct gas so it has 10% ethanol. 
 

I did hit 20 once over 25-30 mile drive but that was with a max speed for around 55-58 on country back roads. 
I always drive with a light foot on the gas and brake. I think my truck has only seen up near redline 2-3 times in 3 years. 

[Chuck FB]

You wouldn't by chance be referring to interstate 94 or 90 would you ? 

 

For sure that is a pretty mild elevation, no its not like driving the coast but a far cry from an extreme of mountain country in Colorado plus the insane altitudes and sucking for air or certainly a naturally aspirated engine is. 

 

I see a few on here or other places on the internet talking about being able to find non ethanol fuel so I wasn't sure, I imagine its a state by state thing and unfortunately here in Alberta with regular I suspect most of it has some ethanol in it and therefore reducing the fuel economy some. 

 

As my truck has that fuel economy readout that records from the moment the key is cycled and then driven and deletes itself when the truck is turned off, its easy to tell by it that when I for example drive back from town to home its starting out with a warmed up drive line and counts up to better fuel economy numbers far quicker which indicates the truck is able to use less power right off the bat due to less friction vs a cold start. While not exactly fair, yes it would be possible to have the vehicle warmed up from a prior distance before starting a mileage run ( no doubt the EPA fuel mileage testing is done just that way to get all they can out of the vehicle to pass ! ) as even that would bump up the mileage test a little. 

 

Not sure how the 6 speed and engine combo is computer controlled compared to the 10 speed but after my truck was past its break in, I did what amounts to a couple of different episodes of a zero to ground speed limit runs to see how the truck reacts as well as anything that might show up by pushing it like one would be doing towing. I did not actually look that closely at the tach but I don't believe it was getting anywhere near the redline before its shifts on its own accord, and boy does it show up as to how the computer pulls power drastically at the moment of shifts to protect the transmission under full throttle, one dead power shift after another and so many shifts as well being a 10 speed which explains why it does not impress speed or time wise in lets say a 1/4 mile. Of course I am little concerned with that, its not a sports car after all !, but am impressed that it is tuned to protect the transmission clutches vs slam shifting. 

[Pryme]

Yeah 94 and 29 mainly. 
 

we have non ethanol fuel here but only in premium and no way am I running premium in this truck. 

[UWSkier]

I've gotten 19+ before, but it's not typical.  I have a drive from northern WI to Appleton tomorrow which is mostly 55 mph roads with small towns.  Wind should mostly be at my back.  Will be interested to see what it returns on that drive as it's also about a 1000 foot descent over the 2 hour drive.

[Chuck FB]

The reason I was curious about the highway is I had been on interstate 94/90 once years ago just driving through from Minneapolis area and on down past Madison and south, its years ago now but in my mind I seemed to think of the drive as quite flat but I covered a lot of ground and its easy to forget certain things. The main problem was the awful road conditions for the first ways from MN due to a winter ice storm as it was in the middle of Dec and miles of traffic going no where in the middle of the night on the freeway but once I got out of that mess the next morning it turned into easy driving with no snow or icy roads to contend with ( other than the touristy pass over the Smoky Mountains ). 

 

I've not taken the effort to test the premium fuel up here and I am told its a bit of a coin toss if it will have some ethanol or not, I only buy small quantities of it for running in chains saws, trimmers and a small generator. But as to regular grade ethanol free, I have never heard its a thing up here but of course was all ethanol free some years ago but leaded fuel instead. 

[Pryme]

Did some digging. Here are some of the cities I went through and the elevations. 
 

eau Claire 797

wausau 1207

stevens point 1100

appleton 790. The airport states the elevation as 918. 
 

That’s over 200 miles so very gradual rise and fall. 

Edited July 31, 2025 by Pryme

[XY74]

That's pretty good.  My 2025 oil burner will get 19 on the highway empty, but that's about it.  My 25 mile best is 24.3, no doubt that was downhill or tailwind.  My 2020 I had a 25 mile best of 54.8.  Obviously that was "cheating", it was driving from Estes Park to Loveland Colorado which is a 2500' elevation loss.  

 

 

[Chuck FB]

1 hour ago, XY74 said:

That's pretty good.  My 2025 oil burner will get 19 on the highway empty, but that's about it.  My 25 mile best is 24.3, no doubt that was downhill or tailwind.  My 2020 I had a 25 mile best of 54.8.  Obviously that was "cheating", it was driving from Estes Park to Loveland Colorado which is a 2500' elevation loss.  

 

 

Speaking of actual realistic fuel mileage and as you pointed out 19 seems to be about the achievable number, then as you pointed out as well with the 25 mile being so easy to manipulate and can give wildly impressive numbers by way of elevation drop or a high tail wind. I had mentioned in another thread of a lady youtuber who claimed her 2024 6.6 duramax gets 27.1 and then lately she happened to scan over her dash just enough that I caught where she came up with the number, it was listed as her best 50 mile. Granted a 50 mile setting would reduce the bs, its still not long distance driving with typical variables. The odds of getting 27 with a 6.6 duramax on a long drive at even a careful highway speed would be quite something. By the way I have been in Estes or as I mistakenly group it together and refer to the area as Rocky Mountain NP as per the loop road and its getting a ways up there all right, that would be a drop from 12000 feet down to 5000 in that case so a 7000 foot drop .. maybe could break the 100 mile per gallon mark !. 

 

Also was going to say that last year I had bought some slightly used factory wheels from a guy who has a 2022 duramax and he was making some rather fantastic fuel mileage claims on his duramax because he had it deleted ( its legal enough in Alberta that shops do it all the time on pickups and highway tractors ) and he claimed there was quite a noticeable jump in the fuel mileage as per numbers like 23.5. However he was going by the dash and I always wonder in the back of my head if a software programmer could if they wanted to, to manipulate that data to make it look even better. Never the less there would be no more fuel thrown out with regens and no more restrictions with all the crap in the exhaust and then tuning/fuel timing would play a part in it as well. I don't doubt there is potential under the hood of these emissions strangled pickups to get better fuel mileage if they reworked them right from the factory, never mind have a fraction of the issues do deal with through the life of the unit that the diesel industry struggles with now. 

[UWSkier]

20 hours ago, UWSkier said:

I've gotten 19+ before, but it's not typical.  I have a drive from northern WI to Appleton tomorrow which is mostly 55 mph roads with small towns.  Wind should mostly be at my back.  Will be interested to see what it returns on that drive as it's also about a 1000 foot descent over the 2 hour drive.

Only managed 17.5 on the dash computer today.  I wasn't lead footing it.  It was windy, not all tailwinds.

[Chuck FB]

11 minutes ago, UWSkier said:

Only managed 17.5 on the dash computer today.  I wasn't lead footing it.  It was windy, not all tailwinds.

Speaking of winds and because of having the fuel mileage readouts, I find that even a stiff side wind or even slightly to the rear of sideways will still knock down the fuel mileage and can be on the same stretch of highway I go on frequently in the same direction. A direct tailwind is great but coming around too much to the side seems to have a negative effect, obviously a head wind is a killer. Still though, getting 17.5 to me is quite remarkable over what historically a truck not this high and heavy in a two wheel drive could dream of getting years back no matter how good the conditions were or how grandpa like one drove ( the proverbial egg under the foot ). 

[Epsilon Plus]

For accuracy, best to hand calculate for those really interested and trusting of their stock DIC read-outs. After 28,000 miles and 3.75 years, my truck has been dead consistently .7-1.0mpg too optimistic (usually .7). If I get 12 the real is 11.3. If I get 11.4 the real is 10.7...etc. Every time.

 

Since my truck is city use only with miniscule highway use, I average 10.5-11.5 mpg depending on whether school is in session and I have to wait in the pickup line. Good thing I barely leave my zip code and only do like 7,000 miles a year or this would be a spendy truck to taxi around town.