A 3 inch size you might not believe is true. Walker's website and this website do not clearly state whether the measurement is ID or OD. I can confirm to you that it is OD. I bought this to replace a broken flange on my 03-05 Ram Cummins. It didn't fit. I ended up welding the broken flange back on so I can't tell you what the correct PN would be for your Cummins.
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