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

[NaturallyAspirated]

FWIW

 

Been meaning to add to the post.

 

I ain’t no fuel expert but every time I drive through the south end of SLC, UT on highway 15 (normally at night and normally it’s cool temps), I get 21 mpg for a stretch that is at least 50 miles long. The DIC even showed 22 mpg on one trip. Always have wondered why since it’s a very flat stretch of highway.

 

Altitude? Low temps? 
I always run 87 octane.

[Chuck FB]

21 minutes ago, NaturallyAspirated said:

FWIW

 

Been meaning to add to the post.

 

I ain’t no fuel expert but every time I drive through the south end of SLC, UT on highway 15 (normally at night and normally it’s cool temps), I get 21 mpg for a stretch that is at least 50 miles long. The DIC even showed 22 mpg on one trip. Always have wondered why since it’s a very flat stretch of highway.

 

Altitude? Low temps? 
I always run 87 octane.

Which direction are you driving and do you happen to know the two approximate points by some reference easy to find on a map ?. Also what time of the year was that as you mentioned a cool temp ?. I know there is a certain amount of elevation change even within the city on 15, well that is if I could ever figure out where the city officially is as it seems to go on forever !. I've only been through it once years ago but also north of and south of the city the elevation goes up at some point as well. 

[EXSlider400]

These trucks don’t really “coast“ very well. Understandably why and look I tow with my truck, but when I’m not towing some sort of economy mode, which allowed for more coasting would be nice (if it’s possible which I am not sure if it is). Anyway, so going downhill or letting go of the throttle really doesn’t help that much. I feel like I need a clutch, but these 10 speeds are so complicated, I’m surely not putting it in neutral when in motion. There’s a spot on the Natchez trace Parkway where I get really good gas mileage, but I’m going 54 miles an hour I might try doing speed limit, which is 50 and see if I can get a little better. I’m going on the interstate if I keep it under 70, I can get around 19.  But I also have a an aggressive all-terrain Goodyear Wrangler Dura-trac RT in the factory 275/65/20 size but they’re still very heavy at 63 pounds each. 

25 mile average

 

50 mile average

[Graves]

I ussually average about 13.5 miles per gallon constantly.

 

[Jettech1]

I can get 19 to 20 no problem on the highway if I have the cruise set on 65.  Here's the wild card that might make me cheating a bit is I have a hard tonneau cover.  From previous questions about this topic it appears a hard tonneau cover will give you more mpg.  How much I don't know but I can easily get 19 to 20 with the cruise set on 65 with my 22 custom model.

[Chuck FB]

2 hours ago, Jettech1 said:

I can get 19 to 20 no problem on the highway if I have the cruise set on 65.  Here's the wild card that might make me cheating a bit is I have a hard tonneau cover.  From previous questions about this topic it appears a hard tonneau cover will give you more mpg.  How much I don't know but I can easily get 19 to 20 with the cruise set on 65 with my 22 custom model.

 

You got me wondering about your truck and being a custom in theory its lighter than a more optioned truck off the assembly line. Of course the cover adds weight but what tire size does your truck have and are they a finer tread pattern than some of the more clunky all terrains out there. Also do yo run high pressures or have them lowered, all of that makes a difference to rolling resistance. What fuel type are you using or if the pump tells you as per ethanol content and also the octane your running. Is that a crew cab short box 4x4 or another configuration ?. 

 

Of course there are other factors, the highway lay of the land, air temp, altitude and wind ( tail wind ) and so on. I still have not been on any longer drive to get a more proper idea of what mine is capable of doing and we have hills and river valleys etc so its never the same as places where its flat. 

[eguzowski]

On 11/24/2025 at 8:23 PM, Jettech1 said:

I can get 19 to 20 no problem on the highway if I have the cruise set on 65.  Here's the wild card that might make me cheating a bit is I have a hard tonneau cover.  From previous questions about this topic it appears a hard tonneau cover will give you more mpg.  How much I don't know but I can easily get 19 to 20 with the cruise set on 65 with my 22 custom model.

I just pulled between 19-20 with my ‘25 3500 High Country gasser over the holiday, about 300 miles each way, all interstate.  I also have a hard tonneau and agree that helps.  I pretty much ran between 65-70 the whole way

[Pryme]

53 minutes ago, eguzowski said:

I just pulled between 19-20 with my ‘25 3500 High Country gasser over the holiday, about 300 miles each way, all interstate.  I also have a hard tonneau and agree that helps.  I pretty much ran between 65-70 the whole way

I also have a tonneau. I believe they do help too. 

[dieselfan1]

Hard tonneau or soft roll up?

Does the soft one flapping in the breeze get less mpg compared to a hard cover?

Asking for a friend 

[Pryme]

15 hours ago, dieselfan1 said:

Hard tonneau or soft roll up?

Does the soft one flapping in the breeze get less mpg compared to a hard cover?

Asking for a friend 

I have a soft but it’s a good one and I installed and tightened properly so it doesn’t  flap. It’s very taught. Rain doesn’t pool on it while sitting. 
 

It’s the truxedo Pro x15. Very low pro and has almost a convertible top material. 

Edited December 4, 2025 by Pryme

[eguzowski]

20 hours ago, dieselfan1 said:

Hard tonneau or soft roll up?

Does the soft one flapping in the breeze get less mpg compared to a hard cover?

Asking for a friend 

My current is the Rough Country hard fold 

[TeamSaris]

On 9/2/2025 at 7:47 AM, EXSlider400 said:

These trucks don’t really “coast“ very well. Understandably why and look I tow with my truck, but when I’m not towing some sort of economy mode, which allowed for more coasting would be nice (if it’s possible which I am not sure if it is). Anyway, so going downhill or letting go of the throttle really doesn’t help that much. I feel like I need a clutch, but these 10 speeds are so complicated, I’m surely not putting it in neutral when in motion. There’s a spot on the Natchez trace Parkway where I get really good gas mileage, but I’m going 54 miles an hour I might try doing speed limit, which is 50 and see if I can get a little better. I’m going on the interstate if I keep it under 70, I can get around 19.  But I also have a an aggressive all-terrain Goodyear Wrangler Dura-trac RT in the factory 275/65/20 size but they’re still very heavy at 63 pounds each. 

25 mile average

 

50 mile average

100% my biggest complaint. even down hilll...it wont coast. my fathers 6 spd truck will though. 

[garagerog]

To my way of thinking, at least from a historical perspective, you're already coasting, come on man 54 mph and only 1200 rpm?  The 10 speed has 3 OD gears, the 6 speed I would guess 2 so there's that. The low mileage '05 K1500 with the 4L60 I inherited from my late father turns 1300 rpm when it goes into 4th OD TC lockup at 45 mph or so. We would always like a 40 mph tailwind and never a headwind but it's not a perfect world on our highways and byways.

[Chuck FB]

1 hour ago, TeamSaris said:

100% my biggest complaint. even down hilll...it wont coast. my fathers 6 spd truck will though. 

 

By any chance do you have cruise engaged when your going down hill ?

[Jettech1]

On 11/24/2025 at 10:03 PM, Chuck FB said:

 

You got me wondering about your truck and being a custom in theory its lighter than a more optioned truck off the assembly line. Of course the cover adds weight but what tire size does your truck have and are they a finer tread pattern than some of the more clunky all terrains out there. Also do yo run high pressures or have them lowered, all of that makes a difference to rolling resistance. What fuel type are you using or if the pump tells you as per ethanol content and also the octane your running. Is that a crew cab short box 4x4 or another configuration ?. 

 

Of course there are other factors, the highway lay of the land, air temp, altitude and wind ( tail wind ) and so on. I still have not been on any longer drive to get a more proper idea of what mine is capable of doing and we have hills and river valleys etc so its never the same as places where its flat. 

Lots of questions here so let me see if I can help you out here.  Let's talk about weight first.  Full tank of gas with me and my wife in it (combined...don't tell her I said this....just kidding...she's a skinny little woman...ok so CAT scale weight is 7800 with just us in the cab).  I have the custom model that came with the 20's on it.  I run them at 65psi not towing, 75 cold when towing to reduce sway.  (Works wonders by the way).  Fuel is the cheapest 87 ethanol I can find, generally Murphy (Walmart) fuel and I have the crew cab.

I don't have the ability like aircraft to monitor tail or headwinds so I have no idea on that question.  Altitude is generally around 1k ft. above sea level.  That changes of course.  Lots of hills and valleys in GA and where we live now in Alabama.  

So to kind of wrap this up a bit, 65mph is the sweet spot for my 22 with the 6 speed.  I generally set the cruise on 72 going back and forth to work these days and it gets mid 15's sometimes low 16's depending on the weather.  If it's cold I always start it up from inside my house to warm it up (I hate getting in a cold vehicle) so that will drop it some as well.

There are a bunch of factors involved when it comes to gas mileage for sure.  I will tell you 93 octane doesn't change a damn thing when it comes to mpg for my truck.  0!!  No change in power, no change in gas mileage, no change other than my account balance...lol....I hope this helped!

 

Dale