Got 32MPG with an automatic '87 300D driving at 65 without any AC. So maybe a little less for you at 75, but that does seem a little low to me. Yes, do check those oil leak(s). Too much.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k