Passenger vehicle sales slumped 23.7% in September - the eleventh straight month of declines - amid one of the worst slowdowns seen in India's auto industry, data released by an industry body showed on Friday. Passenger vehicle sales dropped to 2,23,317 units in September, the Society of Indian Automobile Manufacturers (SIAM) data showed, while passenger car sales dived 33.4 per cent to 1,31,281 units.