Worn or Fouled Spark Plugs
Worn or fouled spark plugs will obviously cause fuel-wasting engine misfires. Platinum and Iridium plugs should last 100,000 miles, but short trip stop-and-go driving may cause the plugs to foul prematurely. An engine that is using oil can also foul out its spark plugs.
Remove and inspect the spark plugs. Clean the plugs if they are dirty, and re gap to specifications, or better still, just install a new set of spark plugs.
Dirty Fuel Injectors
Fuel varnish deposits can build up inside fuel injectors, preventing them from delivering their normal dose of fuel. This can cause a lean air/fuel mixture that results in lean misfires and wasted fuel.
Try adding a bottle of good quality fuel injection cleaner to your fuel tank. It may take several tankfulls before any improvement is noticed. If that does not work, having the injectors professionally cleaned will often restore normal performance. If an injector is too badly clogged to be cleaned, or it is defective, you’re looking at replacing one or more fuel injectors. This may not be a best option because it is not cheap to replace.
If you are driving a high mileage vehicle (over 100,000 miles), you may be getting poor fuel economy because your engine does not have the compression it once had. As the miles add up, so does the wear on the piston rings and valves. This can result in a gradual loss of compression that reduces engine efficiency and fuel economy.
If you suspect low compression, do a compression test on the engine. If low, there is no easy fix other than an overhauling of the engine.
Wrong Oil Viscosity
Most late model passenger car engines today require a low viscosity 5W-20 or 5W-30 motor oil. Some even specify 0W-20. Such oils improve fuel economy, especially during cold weather when the oil tends to thicken. If you are using a heavier viscosity motor oil, it can reduce your fuel economy.
Dirty Air Filter
If your air filter is really dirty, it will interfere with normal engine breathing and hurt fuel economy. Remove and inspect the filter, and if it is dirty replace it with a new one, this shouldn’t cost much compare to the fuel expensis that may be too unbareable.
Clogged Converter or Exhaust Restriction
Any obstructions in the exhaust system will create power-robbing backpressure that also hurts fuel economy. You can inspect the outside of the system for any obvious signs of damage such as a crushed or crimped pipe. But internal problems such as a clogged converter or collapsed muffler or double walled pipe can’t be seen from the outside.
You can check for an exhaust restriction by connecting a vacuum gauge to the intake manifold. At idle, the engine should show a high and steady vacuum reading (say 18 inches or higher). If the reading is less than this or it gradually drops, you have an exhaust restriction.
To achieve maximum fuel economy, your tires must be inflated to the recommended pressure for your vehicle and load. For most passenger car tires, that means 32 to 34 PSI. A low tire increases rolling resistance (and tire wear), and can result in a loss of 5 to 10 percent fuel economy. Check all four tires (when cold) with an accurate gauge, and inflate as needed to the recommended pressure.
A parking brake that is not fully releasing, or a brake caliper that is sticking can cause the brakes to drag and your engine to waste fuel. A quick check for this kind of problem is to park your vehicle on a slight incline, put the transmission in neutral, then release the brake pedal. If your car does not start to roll immediately, the brakes may be dragging.
Poor Driving Habits
This is probably the most common problem and biggest fuel waster of all. Aggressive driving and jack rabbit starts flood the engine with extra fuel. Take it easy, as if you were driving with a raw egg under the gas pedal and you’ll get the most miles per gallon from the fuel in your tank.