In a high pressure fuel supply system, a variable speed motor asynchronously actuates a pumping piston with a drive mechanism including a circular cam with cam roller, having an axis that is offset from the axis of the cam drive shaft. A rolling or sliding cam follower is rigidly connected to the piston, and a piston retainer is operatively connected among the piston, the cam follower and the cam roller. As the cam shaft rotates, the cam roller rotates eccentrically relative to the cam shaft while maintaining contact with the cam follower, thereby reciprocating the follower and the piston along the actuation axis, in corresponding charging and pumping strokes of the piston.