18.5 US gallons, I believe. With about a 2 gallon reserve included in that figure. The later '95 E300 diesel had a 23.8 gallon tank, for those wanting to travel long distances.

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)