In 1945, it was decided to drop the SS name, which had fallen into disrepute during the war, and simply call the company Jaguar Cars. Shortly after the war, sidecar production was sold, and 1.5-, 2.5-, and 3.5-liter sedans and soft top models were introduced to successfully pursue large export deals. The models were called Jaguar Mk IV. The 3.5-litre Jaguar Mk IV proved too wasteful for the United Kingdom, but was ideal for the US, where most of the cars produced during this period were shipped. The SS 100 model was not produced after the war, but one copy survived, not registered during the war.
In 1943, the company's employees Bill Haynes, Walter Hassan, Claude Bailey and Harry Whislake began work on creating their first own engine with a hemispherical ignition chamber. Claude Bailey developed several variants of cylinder head designs. Experimental samples were marked with the letter “X”, the second letter following it (they were added in alphabetical order) denoted the next design: “XA”, “XB”, etc. Many schemes were tried: four- and six-cylinder, overhead camshaft, while on the eleventh letter, it did not become clear that the motor, which received the designation "XK", is what we were looking for. The engine passed a tough endurance test, a 24-hour test where the engine speed was kept at 5000 rpm, and then every two hours the speed was increased for five minutes to 5250, 5500 or 6000 rpm.
Jaguar had a great new chassis, an extraordinarily powerful new engine, but no sports car. The decision was made to release a small number of sports cars to maintain popularity and, possibly, successful racing. William Lyons was faced with the task of developing a suitable body in just a couple of months to take part in the 1948 motor show. The result exceeded all expectations. Known as the XK120, the model was destined to become one of the greatest sports cars of all time. It wasn't just a racing car. The car had the sophistication inherent in Jaguar style, comfort unprecedented for this type of car and, among other things, its price was only £998. The maximum speed allowed the XK120 to become the fastest mass-produced car in the world. To convince the naysayers of this, the standard XK120 set a record of 126 mph on a closed stretch of dual carriageway at Jebbek in Belgium in the presence of the press. With the windshield removed, 133 mph was developed and orders poured in. It soon became clear that the production of two hundred cars could not meet the demand.
In September 1948, Jaguar announced its first post-war transitional model. Circumstances prevented the creation of something more radical, and the Mark V model became the company's glory for a couple of years. The main innovation was the independent front suspension designed by Haynes. By that time, a powerful new engine had been created, but it was decided that the Mark V was too conservative for it, and so the Mark V sedan and soft top model were equipped with conventional 2.5- and 3.5-liter power units. The design of the car had many positive differences. The headlights were smaller and recessed into the front fenders, the overhead door hinges were replaced with hidden ones, the wheels were smaller and only stamped, the Saloon's roofline was sloping and looking more attractive,