OK, please forgive me if I don't see the obvious. Why is exactly does the OP refer to the car as a "Sleeper"?

I'm always understood a "Sleeper" to be a car that doesn't look fast, but is in fact quite fast.

Sincerely,

Packman

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83 240D - 4 speed manual - Manilla Beige
189K miles, Tachometer mod, cool wooden shift knob from PeachParts, CocoMats, Original factory paint, manual windows, manual sunroof. Starting to add AudioWrap to this car too!