SINGAPORE—China’s homegrown new-energy vehicle makers are booming as demand surges, with sales now forecast to hit a record six million this year in the world’s largest auto market. Warren Buffett -backed BYD Co. 1211 -0.89% , which has emerged as a formidable rival to Tesla Inc., TSLA -2.44% more than tripled its sales of passenger cars in July to more than 162,000 vehicles from a year earlier, the China Passenger Car Association said Tuesday. BYD’s own data shows about half of those were pure electric and the rest hybrids. Tesla’s China unit saw sales fall 14% last month from a...