Here is the list of things that will lower your MPG, and what you can do about them.
1) Clogged or dirty air filter: The filter should be changed about every 3 months is the rule of thumb. Remove it from it's box, and shake or bang it against something gently. If alot of dust comes out, is it bad. Some engines even have an air filter guage. If it shows any restriction at all, reset it and replace the air filter. The guage is reset by touching the buttom on the end of it.
2) Incorrectly set ignition timing: This is true especially for engines that have a distributor. Ignnition timing is easiest set, specific to your engine with a vaccuum guage. You disconnect the advance mehcanism, and rotate the distributor to obtain the maximum reading on the vaccuum guage. Higher number (more vaccuum), more air is coming through the engine. This is the best balance between power and economy. Incorrect ignition timing can be caused by other things, too. These things will set an engine light, which will be covered, too.
3)Emmission system problems: The emmissions system is what the computer controls. It is integrated into the design of the way the engine works. There is no 'running situation' that the engine will be in that the computer cannot control. This is what the sensors are for, especially the oxygen sensors. If there is a problem with any component in the emmissions system, an engine light will almost always come on. There are still O2 sensor problems that won't set a code, and the sensor will need to be monitored with a scan tool, while the engine is running. If you have a pre-95 vehicle, you can get instructions for retaining your codes at troublecodes.net. If it is 95 or newer, take it to Autozone or Advance Auto Parts for their free diagnostic scan.
4) Fuel injection system malfunction: This usually means leaking injector(s), or coolant temp sensor problems. The coolant temp sensor tells the computer the temperature of the engine, so it can richen the fuel mixture to warm it up. This is why the engine should properly warm up to the correct temperature. So be sure the cooling systam is in good working order. Injectors are tested for leaks by means of a test called injector drop test. A fuel pressure guage is hooked to the engine and the fuel system is pressurized. Then each injector is fired one by one. When the pressure drops, you have found the leaky injector.
5) Low tire pressure or wrong size tires: Always check the manufacturer recommended specifications for this information. The 'spec card' is in the driver door jamb. There are often different pressures for the front and rear tires, this is for a reason so follow them exactly. People are now filling their tires with nitrogen. This is because it is lighter than air and doesn't expand with the heat range inside a tire. Air does expand, this will make your tires bigger, and harder to turn. I know it doesn't seem like much, but something like fuel economy is built in, and there is nothing that can really be done to change it.
Those cylinders still need the same amount of fuel to work. All you can do is make sure it is burning all of the fuel that is getting put in the cylinders.