1250 rpm is way too low for that kind of load at that speed. The 6.7L engine works best around 2100 rpm, that is what Cummin's list as the best efficient / sweet spot. You need to get the rpm's up, low rpm and high loads will load up the DPF. With a 3.73 rear end, you need to drop the overdrive and think about being in 5th. As for mileage, sorry, but when you are pulling loads, mileage is kinda out the door.

Can the tranny handle a lower gear for a long period of time ? Yes, just watch the tranny temp. You may want to consider a 4.10 rear end, it will place you in the rpm range that will be ideal. However, a 4.10 rear end will lower your mileage overall.

Hope that helps.

Cheers.

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