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We have a 2003 Duramax dual with a 8ft bed. We out a big ass pickup camper on it and then tow a car hauler with a jeep on it.
I had installed an aftermarket exhaust (5") about 10 years ago. but only from the down pipe back. This exhaust is all rusted and cracked and falling off. So looking into replacing it.
And was thinking about replacing the whole exhaust. manifold's on back.
My question is will I see an improvement in mpg enough to pay for it self?
Or should I just put a stock one back on?
We also have a banks aftermarket filter / filter housing on it.
 

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4" would be fine, you won't gain anything manifold back other than bragging rights, as long as your downpipe is still in good condition and you have no exhaust leaks, just leave it on.
Aftermarket downpipe is another part that will not do anything for you, except for lighting your wallet, a lot of labor and again bragging rights. .
You might consider stainless, Diamond Eye has the most reasonable prices on stainless steel exhaust that I've found.
 

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4" would be fine, you won't gain anything manifold back other than bragging rights, as long as your downpipe is still in good condition and you have no exhaust leaks, just leave it on.
Aftermarket downpipe is another part that will not do anything for you, except for lighting your wallet, a lot of labor and again bragging rights. .
You might consider stainless, Diamond Eye has the most reasonable prices on stainless steel exhaust that I've found.
The whole exhaust needs to be replaced from the down pipe on back.

Only reason for thinking about replacing everything including the manifolds was to get better mpg.
 

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Then just replace the exhaust from the downpipe back, manifolds will NOT net anything especially mileage.
Check out Diamond Eye SS exhaust, it really doesn’t cost all that much more than aluminized.
 

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Check out Diamond Eye SS exhaust, it really doesn’t cost all that much more than aluminized.
x2 on going with DE. I had some issues and they were very helpful.

As far as increasing mileage, i seriously doubt... and i mean SERIOUSLY DOUBT... you could find a mod/tune that would increase your mileage enough to pay for itself in less than 5-10 years (likely 8-10 years). An engine can only be so efficient (if you're feeling nerdy google Carnot cycle).

The best bang for your buck regarding increasing mileage: reduce drag. If you can reduce drag even the slightest i promise you will increase mileage (all other factors staying constant, of course).
 

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My mods I actually lost a little mileage compared to stock, which means they will never pay for themselves, but they are good for bragging rights.
Cooler egt’s, a little more power makes for a little smile.

Now being able to have a little better mileage temporarily, a slight down grade really helps. :wink2:
 
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