The paper states how fuel mileage of a passenger car can be improved by increasing the ratio range of its transmission. It discusses briefly what are the practical ways of doing this and why Perbury was chosen. The features of the BL Perbury unit are detailed, together with the test program to which it was subjected. Performance data for the transmission is presented and the control system strategy is defined and the control hardward described. Problems encountered during the test program are reported, together with the solutions adopted.