I have a real puzzler for you!
I have a very well maintained 2005 honda odyssey with 150,000 miles on it. It typically gets 22 m.p.g. It started using an unusual amount of gas - was getting 12 m.p.g. Took it to the mechanic and turned out the gas pump was cracked. We replaced the gas pump and the gas line, and it was getting 14 m.p.g. Took it to another mechanic, and he said that the spark plugs were the problem as they were not official Honda spark plugs (we replaced them at around 110,000 miles). So we installed the official Honda spark plugs, but still no good. I have visited several mechanics and no one seems to know.