The rule of thumb is 200* to 400* difference between the 2 (Post is cooler) I do not know for sure on the 6.7 with the after treatments though. These numbers come from the heavy truck industry in class 7's and 8's. I think that both temps are pretty high to begin with.

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07.5 3500 Quad, SRW, 4X4, Laramie, Leathered, Dark Metallic Gray, 6.7, 68RFE, 20K Reese, Ride Rite's, 2 Viair air compressors, Bully steps, Kleinn Locomotive air horns (5 in all), Billet polished locking fuel door, 'A' pillar mount Auto Meter Pryo, PML X-deep trans pan
06 Durango 285 RL 5'er Lifted by KZ