I thought the DEF was a good addition when it came out. On the GM and Ford trucks the horsepower & torque ratings increased, as well as real world mileage. If I understand correctly the hydrocarbons are being killed in the exhaust instead of choking down the motor with emission components; essentially freeing up the engine for increased power & mileage. Seemed like a no brainer to me, then again I'm not a diesel expert either and may not fully understand the concept.
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