I have a 1978 Cessna 182 which I took out for a cross country joy ride this weekend. This 182 has the 80 gallon tank, with 75 usable and 5 unusable. Before departing, there was 29g in the left tank, and 30g in the right, for 59g total. When I got back, I put 48 gallons in it, which means there was 32g in the tank when I got back (80-48=32). This indicates that I burned 27g during my 2.5 hour flight. This would come out to 10.8g per hour which I am having a hard time believing. Me thinks it has to be a little higher. I'm thinking that I may not be correctly accounting for the 5g in unusable fuel that is somewhere in the plane other than the fuel bladders in the wing. When the tanks in the wing are topped off, do they show 40g in each tank or 37.5 in each tank? (Unfortunately, I can't get out to the hangar soon to check this). Do I use 80g in calculating fuel burn or ignore the 5g unusable, and calculate everything off of 75g usable? I'm confused and am in need of assistance.