The paper presents different cavity wall designs at the rear of a flat-back Ahmed body to achieve drag reduction. The wake balance is assessed using the variance of the base pressure gradient and correlated to the obtained drag reduction. Without the rear cavity, the flat-back Ahmed body is subject to a steady instability producing a strong wake imbalance in the horizontal direction. The low drag rear design consists of a $$5^\circ$$ 5 ∘ inclined spoiler at the bottom (refered to as a diffuser) that first provokes a vertical imbalance and a non-inclined top spoiler with central imprints that re-balance the wake. The drag reduction is 11.5%, the wake structure is observed to respect the symmetry of the body base in both lateral and vertical directions suggesting a full stabilization of the initial steady instability. Both the wake balance and the stabilization are associated with the formation of a longitudinal vortex pair initiated at the imprint edges and penetrating the recirculating area.