I just brought my '13 2500 Laramie 4X4 with the CTD home on Tuesday! I am still learning but from what I have read, under normal city driving and flat interstate travel, the DEF should last quite a while. But, when you start putting a load on the engine, more exhaust gases go through and therefore more of the DEF is used. I am not sure as this is my first diesel but I would have thought it would have last longer than what yours has.
Not much help I know but hopefully someone with more knowledge and experience with this will chime in.
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