The Gazelle was the first Singer to be produced following the take-over of the Singer company by the Rootes Group in 1956. The body style followed by the Gazelle between 1956 and 1967 came to be known as the ‘Audax’ body, with significant input from the US-based Loewy design organization, highly regarded at the time due to Loewy's input to several iconic Studebaker designs which the Gazelle closely resembles.

The main change when the Gazelle II became the IIA was a reversion to the more powerful Hillman pushrod overhead valve engine, developing 56bhp against the OHC’s 49bhp. In line with Rootes Group policy, the car kept getting small upgrades each with a new designation. In September 1958 the car became the III, with improved seats and a two-tone paint scheme. The IIIA of 1959 gained small tail fins and a larger windscreen. The engine was upgraded with twin Zenith carburetors, lifting output to 60bhp. The 1960 IIIB reverted to a single carburettor which improved fuel consumption and facilitated servicing "in remoter terrotories”. The IIIB also received a new back axle featuring a hypoid bevel in place of the former model's spiral bevel. The convertible was dropped from the range in February 1962.