Sherlock’s little ride in Game of Shadows? An 1893 Duryea:
This four-wheeled vehicle has metal tires with a buggy body and top. The one-cylinder, four-cycle, four-horsepower, water-cooled gasoline engine, with make-and-break electric ignition, lies almost horizontally beneath the body, and its cylinder head extends backward above the rear axle.
One cylinder. Four horsepower.
via Just A Car Guy