The low Cd of the W124 body is well known and is probably the main reason that owners enjoy good fuel economy. It follows, then, that anything that disturbs the airflow will reduce mileage. From 27 down to 15 is a 50% reduction, which seems a little excessive (I assume that both quoted numbers were with the veggie system in use). OTOH, I know that a canoe (much slipperier than a bicycle) on top of my car knocks off 2-3 mpg.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970