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I have a similar question. My parents just bought a used 05 2500HD Duramax Turbodiesel. Whoever had it before had just cut off the exhaust maybe a foot after 2 pipes come together. I didn't like that it was getting fumes around the truck at idle. I can't say it was that loud, but it was drowning out the turbo music for my liking so I had my Mom (i'm giving orders to Mom so i'm not a kid lol) take it to a muffler shop and install a 4" pipe the rest of the way out the back to quieten it down and get the exhaust out back.

So here's the deal. The computer was saying we were getting 20-21mpg consistently with no pipe.
After pipe it's saying 15mpg.

Is there that much Backpressure being created with the extra 7ft? The muffler guy told her it would, but I didn't believe it.

What makes this setup so backpressure sensitive? The turbo?

I'm assuming that's why I see a lot of people running stacks. I assumed it was just for looks. Now i'm not so sure.

So i don't know if they make a 6" exhaust pipe, but i'd be willing to try a larger diameter pipe to get it back up to 20. Any thoughts on a 6" doing the trick?
 

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I run a 4 in straight pipe and get 17-20 in the city and 22-24 on the highway (these are actually numbers not what the computer says)
 
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