The pilot/builder of an experimental, amateur-built airplane was in cruise flight at 1,000 feet above ground level when the engine started running rough. The pilot stated that he was unable to maintain altitude, and performed an emergency off-field landing on a road. During the landing roll the tailwheel broke; the pilot then lost control of the airplane, which impacted a metal pole, resulting in damage to the right wing forward spar. According to the pilot, the airplane had 4.5 gallons of fuel on board at departure and flew for 25 minutes prior to the accident. During examination of the airplane, the pilot/builder did not note any mechanical anomalies. The airplane was equipped with a non-certificated, 37.5 horsepower Volkswagen engine.