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Gas milage with an 8.1 Vortec

04-30-2005, 02:17 PM
Just curious what the expected gas milage (without a trailer) would be from a 3/4 ton 2002 model truck (avalanch) with an 8.1 liter engine, 4X4 and 3.73 rear end. I was told it gets under 10 but i think someone was :smokin: too much. I mean its only 2.7 liter biger than the 5.3 which gets great gas milage if driven easy at a constant speed for the size of the truck it is. Just would like a little info from some of yall that have owned one. Thanks

05-01-2005, 08:15 AM
i would say 8-12 mpg thats about what i hear is right ..... you can't forget that its a 496 cubic inch big block

05-02-2005, 07:16 AM
Hey rabt. Can I see a pic of your truck? I think you have the color I've been hunting ever since I've owned my truck.

05-02-2005, 10:06 AM
i would say 8-12 mpg thats about what i hear is right ..... you can't forget that its a 496 cubic inch big block

That's what I hear also. You'll have a hard time pinning a dealer down on a number, due to it's ability to be thirsty with different drivers. I looked at a used 2500HD with an 8.1 in it before buying my 6.0 and the owner said he got 11 mpg once...I passed because it was more than I needed.

When hauling heavy loads, it is possible to get more mpg than a smaller engine with the same load though.

Chevyman, when you state that "it is only 2.7 liters larger", that is more than a whole engine size on some vehicles when you think about it. 496 with an Allison is quite an combination, but in my opinion you should have a specific use for it to buy one. :2cents:

05-04-2005, 11:52 AM
I have an '04 2500HD with the 8.1 and Alison transmission and my mileage averages 12-14 per tank. My overall mileage according to the information center on the dash is 12.9 after 26,000 miles. Mileage seems to drop about 1-2 mpg when I tow a 20 foot bass boat, but I didn't buy this truck for the mileage either.

05-04-2005, 09:24 PM

I have a 2002, hd, full bed, crew cab, 8.1 and the allison. Bought new in Jan 2002.

Truck weighs in at 3.3 tons.

Average mpg = 10. I check the mileage almost every time I fill up.

Have the k&n intake kit, tried the vortex gizmo, run mobile 1 since new.

Truck is all stock. Love the beast, love the power, but the gas mileage sucks.

Best mpg is around to date 12 to 13, but all highway.

Truck runs like factory fresh.

Still searching for the wonder product....

My next move is a tune up (65K and all original).

Read the thread about adding acetone to the fuel.

Seems like its worth a try.

Chevy big blocks are worth the $$

07-26-2008, 09:57 AM
The 8.1 will outrun the 454.You cannot do burnouts with the Allison behind the 8.1,my father tried it with his 2002 GMC dually with the 8.1 and the computer said no.Pulling a trailer,you get about 8 MPGs.

12-19-2008, 11:46 AM
I have an 01 silverado 8.1 with the allison and for the most part i keep my foot out of it. On average i get 11 around town and 14-15 on the freeway but.. i also have a k&n intake, throttle body spacer, 6" lift with 35 BFG mudders and a flowmaster 3' to a 3 1/2 tip. Sounds good, runs like a champ.

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