The benefits I see to the undeside pans are protection from road debis, aerodynamics, and noise reduction. None are really substantial. I am just in the habit of always keeping mine on.

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1996 (W210) E300D 86K - Traded in for a Lexus
1992 (W124) 300D 2.5 Turbo 202K - Sold
1983 (W123) 300D, 146K - Sold
1970 280S, 263K - Sold - Beginning of addiction