01. O2 sensor been changed lately? Check how rich your mix is. I can tell right away when somethings wrong with my mix because my tailpipe gets all sick black looking.
02. Changed your fuel filter lately? I had a clogged filter one time and I thought it was my fuel pump, so AFTER pulling the tank and testing it, changing the filter dawned on me!
03. How fresh is the gas you are using, have you tried different gas vendors?
04. Check your tire balance and wear of tire. It seems when I run a tire to its max life, near the end I get the worst handling, mpg and comfort.
05. PCV been changed lately? When I was running stock with my 91 four door, I changed this little bugger out and I was consistantly getting around 480-500 km a tank. Keep in mind I just replaced back to stock my exhaust and changed the filter to k&n. Since then the kick has a new manifold and exhaust and intake, so now I get what you get 'cause I drive the snot out of it.
06. Get a general tune-up, fluids, and if you have a little bit of cash, replace your plugs, wires and cap and rotor.
Hope some of that helps. I was playing the MPG game for years, and not just with zukes, so don't worry, you'll get it back!
steveo