The Rolls-Royce Silver Ghost had a production run of 6173 vehicles from 1906 through 1925. The company originally dubbed this model the 40/50, after the 40/50-hp six-cylinder engine. The 40/50 tag came from the fact that the horsepower figure came out to 40 for motor tax purposes, which was derived from the cylinder bore dimension using a formula known as the RAC rating. Actual output was determined to be 50 hp, thus the 40/50 designation.
