Diesel Mileage Question

I am thinking of purchasing a 2004 Silverado 2500HD with the Duramax 6.6 and Allison trans. I hear the mileage I should expect is about 16-17 city and
19-20 highway (65-70mph) not towing. Towing is a whole new story - I wouldn't expect much more that 11. Most of my driving will not be towing. When I do tow it is at least 250 miles at a clip.
Are these non-towing numbers mileage numbers in the ball park? Any insight on this truck is appreciated.
Thanks Guys, Dom
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My 2005 GMC Sierra 2500HD with Duramax & 6sp manual trans does 18 city & 20 - 22 highway. The local truck stop I frequent offers premium & regular grade diesel. It doesn't seem to matter which grade I buy, so I get the regular grade as it is cheaper.

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I am getting 16 local, and 18-20 highway. The highway speeds I maintain to keep a high MPG is about 60-65. When I get up to 70+ is get 18 max.
My brother with the LLY 04 is getting 1-2MPG less than I.

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My '04 Chevy 2500HD Crewcab 2wd with the Duramax and Allison gets me 17 during my morning commute (45 minutes, max speed 55) and 20-21 on the highway at 65-70mph. Every mph over 75 or so sees a decrease in mileage (the price you pay for driving a vehicle that big). At 80mph and I'm getting 14-15 mpg. I've never had any long distance sustained speeds over 80 to measure the mpg.
Cheers - Jonathan
--
Jonathan A. Race
Lieutenant, EMS Supervisor
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Thanks for the quick reply. I'm looking forward to the diesel truck.
I take it you are all pleased - I have heard of some problems with fuel injectors.
Dom

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and
That is about what I get in city driving, 2004 Sierra Duramax LB7. Also about 15 to 16 pulling a 5000 pound trailer in the Rocky mountains.
Filled up this morning: $2.20 a gallon!!! - Denver area. Eight miles to FlyingJ where they have it for $2.12.
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I'm just back from a 7000 mile trip hauling a 3200 pound travel trailer with my '04 D-max crewcab w/ Allison. Rig gets 16.5-17.0 towing and 20-22 non towing. Now has 10,000 miles on it. Around town short-trips sucks it down to 16 and slightly below (non-towing of course).
Dom wrote:

-- "Don't undertake a project unless it is manifestly important and nearly impossible."
-Edwin Land
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