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Gasoline direct injection (GDi) fuel pumps use a plunger reciprocating in a sleeve to deliver pressurized fuel. Current industry-standard clearances between the plunger and sleeve create internal fuel leakage rates which are large enough to negatively affect pumping efficiency at low engine speeds. A polymer seal located between the plunger and sleeve has been developed to minimize the fuel leak solving the low engine speed efficiency problem. This paper will present analytical, experimental and validation testing results of improved pumping efficiency attained with this new seal. With the plunger seal, pumping efficiencies are shown to improve by a few percentage points at high engine rpms (erpm) but can double at low (cranking and idle) rpms, without degradation of performance after durability testing.