I think it's a BS Rumor, the reason they went with 6.0 from the 7.3 had most to do with the emission standards coming up. I don' t think there would be a reason for them to go with the cummins in my honest opinion, as the 6.0 powerstroke as it stands right now, has come out ahead in head to head truck tests between the big three. Read the ariticle below for just a bit of info.
Don't get me wrong, I think all three diesels seem to be great! Not for starting a brand war here.
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Steve