unit for retail sale came off the assembly line. The first retail vehicle was delivered to a customer in Denville, New Jersey, on December 15, 2010. GM reported it had built 12,400 Volts in total through December 2011. This includes dealers' demo vehicles in North America and Amperas in dealerships in Europe, crash test vehicles and other unavailable Volts owned by GM. GM halted production for about one month at the Detroit/Hamtramck Assembly plant by mid June 2011 to complete some upgrades, including the installation of new tooling, equipment and overhead conveyor systems throughout the facility. These upgrades allowed GM to triple the rate of Volt production and prepared the plant for 2012 Volt and Ampera production. After the plant retooling, the production rate reached 150 units per day four days a week by August 2011. The Volt plant was also down during January 2012 in preparation for building the California lower-emission version. A four-week shutdown due to slow sales took place between March and April 2012. GM said it had around 3,600 Volts in inventory and needed to reduce dealer inventories as production is expected to meet market demand. GM also extended the traditional two-week summer vacation by an extra week at the Hamtramck plant. GM closed its Detroit-Hamtramck plant from September 17 until October 15, 2012, affecting roughly 1,500 workers on downtime while the plant was retooled to assemble the all-new 2014 Chevrolet Impala alongside the 2013 Volt. This was the second time in 2012 that GM has halted Volt production. Production of the 2013 model year Volt began in July 2012 and customer deliveries began during the same month. In October 2012, GM announced they would build the Cadillac ELR extended-range luxury coupe at the Detroit-Hamtramck Assembly plant, together with the Chevrolet Volt, Opel Ampera, and Holden Volt. The addition of the ELR to the plant represents an additional million investment, bringing the total product investment to million since December 2009. The first 2014 ELRs rolled off the production line in late May 2013. These were pre-production units destined for testing purposes before production for retail customers began at the end of 2013. Deliveries of the 2014 model year Volt began in August 2013. Volt sales in the U.S. reached the 50,000 unit milestone in October 2013, out of more than 60,000 vehicles of the Volt/Ampera family sold worldwide. Production of the 2015 model year Volt ended