BGIII

iTrader: (3)

My 8.1 wouldn't get 17 mpg if it was in park and idling on a flatbed that was driving 70 mph down the road.

Gordo

I've heard claims of better milage with higher octane fuel. And it almost makes sense due to the ecm being able to advance timing with better fuel and the longer/better burn through the complete stroke.
However, I've tried to compare in as identical as possible highway driving habits. In the end, the higher octane fuel might have givin me .1 or possibly at best .2 of a mile per gallon better economy. Problem is, that doesn't make up the difference in cost per gallon for the premium fuel.

Formulabill

Have to agree with Sydwayz . This is a great engine but will never get large mileage numbers. Take care of the motor and you will get many miles of use from it.

Sydwayz

iTrader: (1)

I picked mine up brand new with 3.2 miles on it; in FL and drove it home to MD, 800+ miles right off the lot.

14.1mpg; all highway, bone stock.
(M32/4L85E 4speed Auto/OD with 4.10 rear end--2500 Avalanche)

The HD trucks have either the 5 speed Allison Auto or the ZF 6 speed manual, but the trucks are heavier than the 2500 Avalanche/Suburban; which makes any fuel mileage gain negligible.

akaboatman

iTrader: (1)

I agree. Mine burns fuel by the hour. Or minute LOL. I have a 2002 2500 Hd.8.1crew cab. Reg gas 285000 miles. Great truck. Still strong. Drove a 2008 2500 Hd 6.0 crew cab today on the same trip that takes Me a tank. This truck burned 17 gallons. Not as strong but a nice truck. Artie

befu

iTrader: (2)

2004 3/4 ton suburban - 3.73's. Empty on the highway I get 14mpg, towing a 32' travel trailer (10k lbs) I get 8.25 generally.

Been thinking about picking up another one to tow the 272 Formula with when we go camping. Right now the 99 - half ton burb is pretty maxed out pulling the boat.

Brian

Griff

The best I ever saw with my 8.1 Allison in my 2001 2500HD with 3.73's was 14mpg on the highway. It was usually about 12mpg daily driving.
The best I have seen in my 2007 Classic 2500HD with 4.10's is just over 13mpg and I get about 11.5mpg daily driving. The 2007 has larger tires.
Neither truck ever saw anything but 87 or 89 octane E10 depending on which was cheaper.

Gimme Fuel

Regardless, it is still a very large engine and will never get good mileage but there are ways to make it more efficient. I make people with 6.0 gassers jealous of my 8.1. The 6.0 will do very well with some tuning and airflow changes but they are pigs from the factory. For being 496 cu. in, mine does very well for what it is and you have to take that into consideration. If I drove it or towed it more frequently, then I would be back into a Duramax (I still miss my LMM after I tweaked it). I eventually will swap an Allison into mine and make it even better at pulling. For what diesels cost to buy and run these days, I can put a whole lot of extra gas into my tank.

I also spent a lot of time with the transmission tuning which helped out a lot also. Converter is locked whenever it can, shift pressures are raised and shift times were cut drastically. Shifts nice and firm, not rough, and unless I push the right foot far down, it shifts below 2200 rpm and just used the great torque band this engine has.

Sydwayz

iTrader: (1)

The Allison does not fit between the frame rails of the Avalanche/Suburban 2500 chassis. That's why it gets the Denali transmission with torque management. The Predador Diablo Powertuner removes this torque management, but you will eventually rip the trans out if you abuse it. Mine was a monster off the line when I changed from the Hypertech to the Diablo. With stock tune and Hypertech, it was a dog off the line. With the Diablo, I had to pedal it to keep it from breaking loose, especially when towing the boat.

Check out custom houses that squeeze Duramaxs into the Suburban/Avalanche 2500s for their info on stuffing the Allison in said frame. They have to do a body lift.

Gimme Fuel

I have no problem putting in a body lift to do so. I could also see what it would take to slide the newer 6L90E form the new 6.0l trucks into there also. That may be simpler. I haven't looked into it that much yet. I have removed the torque management on mine also. Made a huge difference. I upped the pressures as much as I could and cut shift times. Seems to be happy so far. Been rodded on and towed heavy for 30k miles now for 140k total miles.