Pushing 75 with the air on gets me 24 with my 87 300D. I never saw 30, but I got close keeping it at 65 with the air off.

Now my wife's 95 E300D, We see 30-37, air on or off, doesn't seem to hurt the mileage.

My 87 300 TD, (now my daughter's) we have seen 31 if you keep it below 70.
Bottom line, take your 27 to the bank, unless you want to slow down.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes