On July 5, 2007, about 1630 mountain standard time, a Saunders Stiletto, N337DS, experienced a total loss of engine power during cruise flight and the pilot made a forced landing on Interstate Highway 10, about 25 miles east of Quartzsite, Arizona. During flare, the experimental homebuilt airplane impacted a vehicle and was substantially damaged. The airplane was owned and operated by a private individual. The commercial pilot was not injured, and the passenger sustained a minor injury. Visual meteorological conditions prevailed at the time of the personal flight, and no flight plan had been filed. The flight was performed under the provisions of 14 CFR Part 91, and it originated from Chandler, Arizona, about 1545. The pilot reported to the National Transportation Safety Board investigator that earlier during the day his airplane had been serviced with fuel at Chandler. The pilot stated that he did not confirm that the right wing's fuel cap had been secured prior to taking off on the accident flight. While flying en route, no evidence of any mechanical malfunction was noted. Suddenly, all engine power was lost and the pilot made a forced landing on the highway. The airplane impacted a vehicle during landing. The pilot reported that he subsequently inspected his airplane and observed that the right wing's fuel cap was missing. The pilot stated that fuel in the tank had evidently ported overboard during flight. The pilot indicated that the accident could have been prevented had he performed a better preflight inspection.