06 E320 CDI large reduction in diesel fuel Economy

have noticed a large drop (maybe 20% or more) in diesel fuel economy after 1st year. Not my imagination. was getting over 30 around town and almost 40 HWY. now 24/30. Has anyone else experienced this and
what is the cause???
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hard to believe those numbers in the first place, my o5 has always gotten 26/34 averaging 30 mixed daily use of 66k miles
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