Hi, I've also got an 85 300D, California model, that is kept in pristine shape. All maintenacence is kept up, and I drive all highway, approx. 3000 miles a month! The absolute best I've gotten was 26mpg. It makes little difference between city/highway. However, if I use the A/C, my milage drops to 20-21 mpg. So if your using your a/c and still getting 23 mpg, your doing great!

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2012 550CLS
1982 380SL
1985 300D
1998 ML-320
1997 E-320
1984 380SE