A sleeper is something/someone unassuming or unnoticed whose value ultimately exceeds generally low expectations. E.g., a race horse might be ignored by bettor's in favor of flashier or winninger horses, thus attracting long odds, but a few notice his pedigree is perfect for the race conditions and call him a sleeper. So, a car that is a sleeper could be one that is faster than it looks, but it could also be one that is more reliable than it looks, or more comfortable than it looks, or capable of being prettier than it's original state ... etc. ... it has value not everyone can appreciate at first glance.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)