While on an extended base leg the airplane's engine lost total power. The airplane crashed just short of the approach end of runway 31. The on scene investigation revealed the airplane's fuel tanks were empty. The 2 day cross country was planned from West Mifflin, PA, to Hatteras, NC, returning to West Mifflin on the following day. The pilot stated the total flying time for this cross country just prior to the engine quitting, was 6.25 hours. The pilot stated he thought he had 90 gallons of fuel on board before he started the cross country. The pilot stated the airplane's gauges also showed the airplane's tanks were full prior to the departure from West Mifflin. The pilot stated that the analog fuel quantity gauge indicated, just prior to the engine quitting, the left fuel tank had a few gallons left in it and the right fuel tank contained approximately 10-12 gallons. Cessna performance charts show that fuel consumption could have ranged from 12.5 to 16.1 gallons of fuel per hour.