My 05 Duramax with the Banks upgrade got 20-21mpg on the highway running 70-75 with stock wheels no trailer. When I put 20x9's with extra width rubber it dropped to 18-19 no trailer.
My 01 8.1L got 10.0 unloaded at the same speeds. After I upgraded it to Hypertech Programmer only, it went up to 11.5mpg. I had to have one heck of a tailwind to get any better. Towning a trailer or the boat it dropped to 7mpg. I never had an oil issue with my 8.1 and it usually needed a quart when it was time to change the oil. I did put 230,000 miles on it and it still did not use oil.
Both of mine have been the 5 speed Allison and I love it. I did grenade one in my 05 with 120,000 miles.
Thank god for sponsorship from the dealer. They put a new one in for $4400.00 out the door.
All other dealers wanted $7000.00-$8500.00.
If was was buying another new one this year I would do the Duramax again-no questions asked!!!!
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Chris
1967 Yenko Stinger YS-199
Southeast of Disorder