DETROIT, Michigan - General Motors will know by May or June, 2012, if its plug-in electric Volt will succeed in the market.Product planning chief Stephen Girksky said at the 2012 Detroit auto show: "We'll know by June if this car is gonna have legs or not. We are prepared for it if it does."GM has sold 8000 Volts since its launch just over a year ago; the forecast was 10 000 but Girksky said the primary constraint had been short supply.SHORT SUPPLYDespite low volumes, the Volt had paid dividends in terms of helping the once-struggling automaker to attract customers and to burnish its green credentials. The car was the first plug-in to reach the US market and had attracted a lot of wealthy customers to GM.Girsky noted "it's bringing more BMW customers to GM than (luxury mark) Cadillac these days".If Volt sales don't take off then GM has a number of other green cars to offer from highly efficient traditional petrol engines through to natural gas to diesel and hybrids."We want to have a range of product to provide our customers," he said. "People perceive us as a truck company. That's changed in 2011 and there's more to come."'NEED TO BE RELEVANT'GM's compact Cruze sold more than a million worldwide in 2011; prototypes of several more small cars are on show at the Detroit auto show."In the long term," Girsky said, "we need to be relevant for small cars.Environmental concerns and rising fuel prices, he believed, would become more pressing issues in coming years.