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6.6L L8T gas engine experiences

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148K views 54 replies 30 participants last post by  gofast2001  
#1 ·
Hi,
How is your experience with L8T sierra engine?
1. How is reliability?
2. MPG? Avg mpg? Hwy mpg ?
3. Do you experience high oil consumption per 6k miles, even when not towing?
4. Any issues with knock sensor?
5. Do you believe L8T can last 200k miles without major breakdown/repair? Of course assuming one is taking care of service intervals etc.

Deciding whether L5P or L8T.
L5P seems to be great but those regens...
Thanks !
Bob
 
#11 ·
He must have a heavy foot. I get about 8-9mpg towing a 11k toy hauler, and 14mpg on my commute to work, 2 lane backroads. Got 21mpg ,not towing, on a long highway trip a while back.

Haven't had any issues with mine, in the roughly 2 years of ownership. Knock on wood.
 
#7 ·
I only have some experience with the 6.0 on my current HD rig. I have a 6 inch lift with 35's and 4:10 gears and I get 10 mpg combined and about 14 highway. I believe the 6.6 would much better overall in a truck with 4:56 or 4:88 gears instead of 3:73's. I wanted to trade my 2011 chevy 1500 for a 2020 but it would have been too much of a monthly payment for me. That's why I went with my current 2015 GMC Denali HD 6.0 4x4.
 
owns 2015 GMC Sierra 2500HD Denali
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#8 ·
2022 2500HD with a little over 11,000 miles. I pull a 30’ camper weighing 8000 lbs loaded from Florida to Tennessee. Get around 9 mpg towing and 13.5 around town. Don’t even know the camper is back there. Only truck I’ve ever had that I can use cruise control towing without over revving of the engine up hill and drastic downshifts. Had a Dodge 2500 and will never go back.
 
F
#14 ·
I have a '22 GMC 3500 4x4 long bed with the 6.6 gas. I has been flawless. I have a cab over camper and a trailer. When towing with a lot of weight I get down to 10-11 mpg. Empty, I get a consistent 14.5 mpg.
I love this truck. It tows like there is nothing behind it. If someone is getting 10 mpg empty, something is wrong with their driving style or their rig.
 
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#18 ·
Ill add my info, i have so far.
5,200ish miles on my 2024, regular cab, 3,500 4x4 6.6 gasser. WT.
It is flat and rural hear. So far its a solid 14.5-16mpg, per tank. Fuel hear is also 10% ethanol. Im sure that isnt helping. Not sure what summer and summer/warmer temps fuel will bring, as its about 2 or more months away hear.
A month ago, i took a trip to visit a friend, 958 miles one way, farther south than hear in upper MN. Interstate speeds 65-80mph. I go 2 over. Mostly the speeds were on highways 70, 75 and 80.
At that time it had probably 1,200 miles on it. 12.5 worst in the 80 with a fair headwind, then a good 16 when it warmed up more south and no ethanol fuel as well as 70 speed limit.
A fair 14-15 mpg per tank on the way back north.
In about 2 months as mentioned, when/if warm yet, ill be getting a new used 5ver i bought. The 5ver is empty but will be estamated at 8,000lbs. The 60 mile trip to the campground wont be a very good test, but 55, 60 speeds to interstate will be 65mph, on the trip.
I bought the cheby to go back to full time rving. Ill post a pic. New drop in hitch, and new 45 gallon gas approved/legal, tank in bed.
Image
 
7
#19 ·
Just joined, thanks for having me. Here to learn a little about my 2024 GMC 2500 with the 6.6 gas engine and the truck in general. I retired and this is only my fourth GM product in 52 years of driving. A 1970, a 1964, a 1974 now this beast. :rolleyes:
I am not tech savey at all. The ding, dings, the horn honk when I shut it off and open the door, the headlights come on when I push the start button and stay on after I get out. I hate this stuff, as well as fobs, I detest fobs, and last but not least no gas cap. But all makes are following suit. I was a Toyota guy forever, a few Fords, But sinceToyota walked away from a v-8 , I walked away from Toyota. Except my SUV :D But at least it has a gas cap.

So far 5280 miles as of yesterday I am pretty happy with my 2500. 9.9 to 19.1 mpg, tank to tank, so far, average via computer is 14.1. I have towed a 9000# skidsteer on a 18' car hauler with a Scout camper in the bed, 1100#. 300 miles. That one was a tear jerker., 9.9 average. But that included 90 miles over the Big Horn Mountains, 70mph on interstate and 55 in mtns. Yesterday, another 200 mile mnt trip with 18' trailer and a friends 6 passenger side by side. 12.4 mpg. It will go as fast as you dare on steep grades. Open highway, 60 to 65 mph, with about 500 miles on it was the one and only time I got 19 mpg tank. Most of the time if I stay out of it, on the highway I can get 14 mpg on 100 to 320 mile trips. I am old and retired an in no hurry.

My only issue besides the tech BS,:poop:, I did my first oil change, myself, oil and filter. I changed it because I couldn't detect any oil on the funky dip wire, so I changed it. I know 8 qts, there was oil in it. So, I monitored it, at 4900 miles oil barely visible on the wire. I was going to wait until 5500. Dealer told me 10k on oil change, :eek:no way will I ever do that. I use Mobil 1, truck and suv 5-20. My Tundra was 0-20 in the end, before trading, it was starting to use a quart per oil change. I don't like this. Not sure what to think with this truck. I am thinking 4000 mile changes since it is easy and old school. And not terribly expensive by todays standards.
 
B
#20 ·
2022 HD 6.6 Gasser. 21k miles, 35" Nitto, no lift. I'm in Michigan, took a trip to Colorado and averaged 12-13mpg. Towing a boat, averaged 9.9 mpg. It tows great! I'm amazed at how well it tows. I used to drive a Dodge Cummins, and this engine tows almost the same as the diesel.
I go the speed limit, towing or not.
The only issue I've had is oil changes at the dealer. As some have mentioned, I don't wait for 10k miles, I'm 4-6k miles max oil change. Every oil change I've had, the dealer claims and oil pan leak. I've looked, and while there is a bit of oil near the pan, it's never enough to actually drip. It seem to come from the front of the pan/engine where it is open (I'm not well versed in engine terminology). I've talked to GM about it, they will buy it back, but I'm not interested in that. I love the truck and have way too much $$ in extras..Borla cat-back, ceramic, etc..
 
F
#21 ·
Have you been checking what the oil life monitor (OLM) is indicating at the time you are changing your oil?

It doesn't 'test' the oil with a senser.
The number is calculated based on how the engine is being used -- but it will also countdown even if not being used -- so the maximum time before it recommends a change matches the recommended maximum in the owners manual.

I do have a tech background and was skeptical when the OLM's were introduced (in the '80's I think).
At the time my driving pattern was always the same and was changing on my usual schedule, but making note of the OLM status.
My driving pattern changed -- and much to my surprise, so did the OLM recommendation.

Anyway, I now use the monitor - but I also do oil changes to suit my schedule. I don't want oil change to interfere with my busy season, so if it appears that the OLM is going to recommend a change during that period, the oil gets changed early so that interference doesn't happen.
 
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#34 ·
Its flat hear and i drive moderate. After what seemed like 10,000 miles my 6.6 gasser, 2024 3500 broke in, as well as spring hit and no winter blend gas fuel. ( yes, there is winter gas and diesel fuel thanks to the guberment ) Around 18,000 miles on it now. I dont do much stop and go but some is with each tank, rural speeds 57-62, sometimes 67 and 78 on the interstate. A solid average is no less than 16.5ish per tank, best about 18 once, with 87 ethanol. I can live with that as its really a big wind sail with big stock tires, a bit heavy also. Its nice to just run 87 octane. IF one wants to call it that. Hear up north its hard to find non ethanol fuel. I tried 2 tanks of 91 at about .55 more per gallon and netted 1 more mpg, so not worth it. I was able to find regular 87 and ran 2 tanks, at that time it was actually cheaper at that place than my normal place forced to get ethanol blend, it noted a solid 1.5-2 more mpg without the ethanol. All winter diesel was a solid $1.35 more per gallon than gas, and still is at least .50 cents more than gas. My driving comparing my past diesel to the new gasser is .50 cents a gallon tank to tank cost, the gasser getting less fuel economy. Not including def and the cost of the motor for the diesel.

I fulltime rv it, so the camper is home. I tow a simple 10,000lbs 5ver, for about 4,000 miles total a year then the pickup is a car the rest of the time. I had a diesel 2500 for the last 8 years, and short trips arent best for a gasser, and really arent good for a diesel. Most of my driving is short trips.

The simplicity of the 6.6 L8T is amazing after having a turbo diesel, a 2015. Its seems to like to pull/tow, it adjusts per load and headwind, speed and such, picks a gear and does its thing.
 
#39 ·
I think 16-17mpg solid tank to tank for me is great for a 3500 gasser. Thats 55-60 speed limits areas. On the interstate going 70-80 its not good, but neither was my past diesel pickup at those speeds.
If i can find it for a comparative price, non ethanol fuel in 87, i gain a solid 1.5-2mpg.

Cost mile per mile, past diesel vs new gas pig, .50 cents a gallon diesel being more than gasoline, is the brake even point for my driving. Diesel was $1,35 more a gallon all last winter than gas. Since i bought the gasser, diesel has been a solid .40 cents up to the said $1.35 more a gallon, in areas i was/am in.

Same with towing my 5ver, i get around 2mpg less that in did with the diesel, and i was only towing a TT with the diesel, so cost is about the same. Less the cost for DEF fluid, let alone the motor cost.
 
#40 ·
lineman1234 said:
I think 16-17mpg solid tank to tank for me is great for a 3500 gasser. Thats 55-60 speed limits areas. On the interstate going 70-80 its not good, but neither was my past diesel pickup at those speeds.
If i can find it for a comparative price, non ethanol fuel in 87, i gain a solid 1.5-2mpg.

Cost mile per mile, past diesel vs new gas pig, .50 cents a gallon diesel being more than gasoline, is the brake even point for my driving. Diesel was $1,35 more a gallon all last winter than gas. Since i bought the gasser, diesel has been a solid .40 cents up to the said $1.35 more a gallon, in areas i was/am in.

Same with towing my 5ver, i get around 2mpg less that in did with the diesel, and i was only towing a TT with the diesel, so cost is about the same. Less the cost for DEF fluid, let alone the motor cost.
Click to expand...
This is why i stick to the gas burners..
 
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