It incorporated the propulsion system from the Chevrolet Volt, including the Voltec powertrain. In August 2011, General Motors announced it would produce the Converj as the Cadillac ELR. The first 2014 ELRs rolled off the production line in late May 2013. These were pre-production units destined for testing purposes and production for retail customers started at the end of 2013. The ELR was released to retail customers in the U.S. in December 2013. Volt MPV5 At the 2010 Auto China show General Motors unveiled the Chevrolet Volt MPV5 Concept. The Volt MPV5 is a plug-in crossover hybrid and has a top speed of and an electric range of . The MPV5 integrates design elements from the Volt, with a body style very similar to the Chevrolet Orlando and four inches larger than its predecessor, Chevrolet HHR. Opel Monza Concept The Opel Monza Concept is a four-seat coupe plug-in hybrid concept car with gullwing door unveiled at the 2013 Frankfurt Motor Show. The concept shares the same basic plug-in hybrid setup as the Chevrolet Volt and Opel Ampera, but using a turbocharged 1 L 3-cylinder natural gas-powered engine as its range extender instead of General Motors’ current 1.4 L gasoline engine. According to Opel, this concept is the role model for the next generation of Opel cars, and because of its modular chassis design, future cars based on it would be able to accommodate gasoline, diesel or electric power. Controversies and criticism EPA fuel economy testing In 2008, General Motors was concerned about how the United States Environmental Protection Agency (EPA) would test the Volt to determine its official fuel economy rating. The controversy centered on whether, by including a gasoline engine, the Volt should be classified as a hybrid rather than an electric car as claimed by General Motors. If tested with the same EPA tests used by other hybrids, the Volt's EPA fuel economy rating would be around due to the current EPA test for hybrids disallowing vehicles from boosting their mpg rating using stored battery power. General Motors stated that the Volt is an entirely new type of vehicle that the EPA's current fuel economy tests are not suited to rate and that a new test should be devised for this emerging class of hybrid-electrics. General Motors also advocated for a more simplified mpg calculation method to take into account the range of a plug-in hybrid while running