Production hold-ups at key manufacturing plants helped to push General Motors’ US sales in May down 18 per cent, against a backdrop of concerns about how long the boom of recent years can last. Te slide comes as the seasonally adjusted annual rate of sales (Saar) — a key measure of US demand for all carmakers — is expected to be around 4 per cent down from last May, at 17m vehicles. However, the figure remains close to the 17.5m record sales rate in the US in 2015. Sales levels had been expected to be lower because there were fewer Saturdays and Sundays in May 2016 than in the same month last year. GM’s decline sent its shares down 3.39 per cent to $30.22 by the close of trading in New York. Shares in Ford fell 2.82 per cent to $13.11 after it posted a 5.9 per cent year-on-year decline, while Fiat Chrysler shares lost 1.81 per cent to $7.04 in spite of reporting a 1 per cent sales increase. Japan’s Toyota, number three in the US by sales, reported sales down 9.6 per cent.