I guess it was a good thought by the engineers, but having to get it to 30 m.p.h. necessarily requires another vehicle to push/pull it, which for most purposes vitiates the reason for the pump to begin with. I guess that's why they discontinued it.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles