The pilot stated that he filled the amateur-built Challenger II airplane's 10-gallon fuel tank before departure, and he uses approximately 3.3 gallons per hour (gph) in his fuel consumption calculations. After takeoff, he flew for about 1 hour, and then elected to return to the departure airport. At that time, the fuel quantity gauge indicated "1/2" remaining. The flight continued, and at an unknown time, the fuel quantity gauge indicated "1/4" remaining. He diverted towards a nearby airport, but the engine experienced total loss of engine power. While descending for a forced landing, the airplane collided with a tree. Examination of the airplane by a Federal Aviation Administration inspector revealed the fuel tank was empty. It was not breached and there was no evidence of fuel leakage. The Rotax "Operators Manual" indicates fuel consumption to be 7.0 and 5.4 gph at takeoff and 75 percent continuous power, respectively.