The pilot reported that prior to arrival at the airport he checked the weather, which was reporting that the wind was variable at 7 knots. While on final approach, over the numbers, he reported that the airplane was not descending as it should and seemed fast. The pilot observed the windsock straight out and realized he was landing with a tailwind. As he was deciding whether to go around or not, the landing gear touched down, and he decided to make a full stop. The airplane subsequently continued off the end of the runway and over a cliff coming to rest upright about 120 feet from the runway in an open dirt area. The airplane sustained substantial damage to the engine firewall and undercarriage. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.