The engine is a 155 hp / 4000 rpm 4.9 L (300 cu in) six-cylinder unit with inlet over exhaust valves: twin SU carburettors were added in September 1957.

[2] Brakes are hydraulic and assisted by the Rolls-Royce mechanical servo with 11 in (279 mm) drums and suspension was independent coils at the front and semi-elliptic springs at the rear.

A long-wheelbase version lengthened by 4 in (102 mm) was also made available in September 1957, outwardly very similar to the existing car but offering improved leg space for rear-seat passengers.

[5] The new wet-linered V8 was also a little cramped in an engine bay intended originally for a narrower unit: in order to change the spark plugs it was necessary to remove the front wheel on the car's right side.

[10][note 1] External dimensions were slightly altered, the interior remodelled, the weight reduced by a little over 100 kg (220 lb) and improvements made to the engine which included fitting 2-inch (51 mm) SU carburettors in place of the 1+3⁄4 inch units used on the Series II Silver Cloud.

Other external changes included a slightly increased slope of the bonnet to correspond with a 1+1⁄2 inches (3.8 cm) reduction in radiator grille height.

Stainless steel wheel trims replaced chrome-plated ones in April 1963, and an improved rear window demister was introduced in November of the same year.

[5] Wider front seats were fitted in January 1964, and five months later a revised headlamp surround now incorporated a very small RR monogram.

The unusual car, for a working police detective, featured in many show storylines, which included efforts by Burke to seduce females tangential to the weekly murder.

[12][13] A black 1965 Silver Cloud III 'Chinese eye' Mulliner Park Ward drophead coupé previously owned by Jimmy Savile was prominently featured in the 1966 Michelangelo Antonioni film Blowup.

[16] A white 1965 Silver Cloud III featured prominently in The Avengers episode "Mission: Highly Improbable" broadcast November 1967.