I average 21 to 22mpg from my 85 300D. I think only the e-bay 617 engines get 27mpg! Winterised diesel drops mine to 18-19. It sounds like normal milage to me. If you drive it like you stole it that is...

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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