Certain Special Equipment roadster and fixed head coupe cars were produced were sold between 1948 and 1949 as an early production build for enthusiasts, denoted by an 'S' preceding the chassis number.

On 30 May 1949, on the empty Ostend-Jabbeke motorway in Belgium, a prototype XK120 timed by the officials of the Royal Automobile Club of Belgium achieved an average of runs in opposing directions of 132.6 mph (213.4 km/h) with the windscreen replaced by just one small aero screen and a catalogued alternative top gear ratio,[note 1] and 135 mph (217 km/h) with a passenger-side tonneau cover in place.

The DHC and FHC versions, more luxuriously appointed than the constantly exposed open cars, had wind-up windows and wood veneers on the dashboard and interior door caps.

The dual overhead-cam 3.4 L straight-6 XK engine was highly advanced for a mass-produced unit of the time, featuring a high-temperature, high-strength aluminum alloy cylinder head, hemispherical combustion chambers, inclined valves,[9] and twin side-draft SU carburetors.

[2] Most of the early cars were exported; a 7:1 low-compression version, with commensurately reduced performance, was reserved for the UK market, where the post-war austerity measures then in force restricted buyers to 70 octane "Pool petrol".

The Jaguar factory's access to 80 octane fuel allowed it to provide cars with the higher compression ratio to the press, enabling journalists to test the model's optimum performance in Belgium, on a long, straight stretch of road between Jabbeke and Ostend.

The drophead coupé (DHC) had a padded, lined canvas top, which folded onto the rear deck behind the seats when retracted, and roll-up windows with opening quarter lights.

On cars fitted with optional centre-lock wire wheels (available from 1951), the spats were omitted as they gave insufficient clearance for the chromed, two-eared Rudge-Whitworth knockoff hubs.

[15] XK120s were active in racing and rallying: In 1950 Jaguar allocated six alloy-bodied XK120s to drivers Leslie Johnson, Peter Walker, Nick Haines, Clemente Biondetti, Ian Appleyard and Tommy Wisdom.

[22] Details of the model were included in an "Advance Particulars" brochure for the XK[23] but Jaguar's managers were dissatisfied with the engine and the project was cancelled prior to production.