Full scale heavy vehicle aerodynamic testing requires a very large wind tunnel test section, with few wind tunnels having this capacity worldwide. Small scale testing often requires a loss of model detail as well as introducing Reynolds Number and compressibility effects. A ¾ open jet wind tunnel set-up has been developed at Monash University Wind Tunnel that enables testing of 1:3 scale truck-trailer models, of full-scale length up to 18 metres to be tested.The measured drag on longer vehicles is more strongly affected by horizontal buoyancy and long models create additional blockage when yawed. In addition the length of the model means that special care must be taken to ensure that shear layers emanating from the nozzle at the start of the test section are sufficiently separated from the shear layers and wake at the base of the truck.In preparation for 1:3 scale testing the wind tunnel test section was lengthened, the jet size was increased and a false floor was added to reduce the size of the ground boundary layer. This paper details the modifications made and the following validation study undertaken. Presented are streamwise static pressure profiles, boundary layer evolution profiles, measurements of the growth of the jet shear layer and flow mapping planes to establish flow uniformity.