James Taylor suggests four reasons why Jaguar boss Sir William Lyons might have decided to add yet another model to an already extensive Jaguar range:[2] Consequently, Lyons initiated development of a new saloon based on the S-Type, retaining its 54-inch independent rear suspension but adding a twin-carburettor version of the 4.2-litre powerplant and frontal styling more akin to that of the Mark X.

Compression ratios of 7:1, 8:1 and 9:1 could be specified according to local fuel quality, the difference being obtained by varying the crown design of the pistons.

The factory-quoted horsepower rating of 245 bhp (183 kW; 248 PS) was measured using the SAE (gross) system current in the USA at the time the 420/Sovereign was sold there.

A novel mechanical feature that the 420/Sovereign shared with the 420G was Marles Varamatic power steering,[4] which was offered as an option on the 420 but was standard on the Sovereign.

The rise in gearing (equivalent to a drop in ratio from 21.6:1 to 13:1) occurred almost entirely within the first half turn of the steering wheel from the straight-ahead position.

Other mechanical refinements the 420/Sovereign had over the S-Type included: Contemporary road tests[5] indicate that the performance of the 420 and Sovereign was very highly thought of.

A Motor (UK) road test in May 1967 reported: It seems somehow insolent to apply medium [price] standards to a saloon that for a combination of speed, comfort and safety is as good as any in the world, regardless of cost.A North American perspective was provided by Road & Track, whose December 1967 report concluded: Jaguar's big seller in the U.S. remains the E-type sports cars, but the 420 sedan offers just as unique a combination of qualities in its own field.

For a reasonable basic price of $5900, Jaguar offers brisk performance, outstanding braking, excellent handling and ride, quality finish, and luxury in abundance, all in an automobile that's easy to maneuver in today's maddening traffic.A road test by Wheels (Australia) in August 1967 enthused: While they can continue to build and sell cars as good as this, there is hope for the man who cares about his motoring.

In the Daimler model range, the Sovereign filled a gap between the 2½-litre V8 and the larger and more conservatively styled 4½-litre Majestic Major.

In total, the Daimler differed from the Jaguar in having: Difficulty in differentiating the 420/Sovereign from other Jaguar/Daimler models has meant that they are less well known than other Browns Lane products of the era.

The Motor magazine of October 1966 referred to the 420G as "still one of the best looking large cars in the world today" and commented on the similarity of its new radiator grille to that of the 420.

[2] Nevertheless, relatively few were made in total due to the fact that the Coventry factory stopped making the Jaguar 420 in 1968, just over two years after production began and with just 10,236 produced.

Introduced late in 1968, the XJ6 was slightly larger than the 420 and swept it from the Jaguar range along with the Mark 2 and S-Type, although the Daimler 250 remained in production into 1969 and the 420G lasted until 1970.

The decision by Sir William Lyons to base the Jaguar XJ6 on the engine, suspension and approximate dimensions of the 420/Sovereign showed his faith in the 420/Sovereign formula as the best way to rationalise the company's saloon car range.

Rear independent, with lower wishbone and driveshaft as upper link, radius arms and twin coil springs with telescopic dampers