The pilot reported that he departed with 30 gallons of fuel onboard. He estimated that the right and left fuel tanks contained 20 gallons and 10 gallons, respectively. The pilot reported that the accident flight was 2 hours 20 minutes in duration. While on final approach the engine lost power and the airplane impacted a 6 foot wide drainage ditch. The airplane sustained damage to the left wing, the lower fuselage structure, and the nose and left landing gear. Post accident inspection of the airplane showed that the right tank was empty and there was fuel present in the left tank. The pilot reported that he planned on a fuel burn rate of 8-1/2 gallons per hour. Using that fuel burn, the calculated fuel consumption for the accident flight was 19.8 gallons. The pilot said the accident could have been prevented if he had switched fuel tanks before the loss of engine power.