His most significant success was in the large number of heavy-duty trucks he manufactured.

The four-wheel, chain-drive Jeffery Quad eventually became the workhorse of the Allied Expeditionary Force.

[3] It was sunk after being torpedoed by a German submarine off the Irish coast, with a heavy loss of life.

Jeffery survived, but he lost interest in the company, or as one author put it, "re-evaluated his priorities".

[citation needed] During his lifetime, Jeffery collected an extensive library of rare books, maps and autographs which was sold at auction following his death at Merion Station, Pennsylvania.