[50] In partnership with the United States Department of Energy (DOE), National Aeronautics and Space Administration (NASA), and built under
contract by AMC's AM General wholly owned subsidiary, the United Stirling AB's «P - 40» powered Spirit was
tested extensively for over 50,000 miles (80,467 km) and achieved average fuel efficiency up to 28.5 mpg ‑ US (8 L / 100 km; 34 mpg ‑ imp).