As a data point, on my 98 I averaged 26.5 mpg over the course of 12,000 miles. 30.6 was the best I ever got.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 161k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 147k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 81k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife