When I was an ops agent for Alaska Airlines I was always dealing with the fueling and fuel burn of the MD-80s and 737-200s and 400s. The 400 was the most efficient (they didnt have 700s or 900s when I was there) but not a whole lot more than the MD-80. On a typical nonstop FAI-SEA flight of approx 3hr 30 min, a 737-400 would burn about 20,000-26,000lbs of fuel; an MD-80 would burn 22,000 to 28,000 pounds of fuel. So, they are comparable. One thing to note, the MD-80 could carry more cargo/bags/mail than a 737-400 and it was MUCH to perform weight and balance on when full. So, sometimes when a 737-400 was at its MGTOW, it would be IMPOSSIBLE to get the CG within limits, however an MD-80 at MGTOW was impossible to NOT be within limits since the CG was soooooo wide. Just a note comparing the two!