"Bad Gas Mileage"
With the fluctuation of gas prices these days, you'd be crazy not to want to improve your gas mileage. below I list some reason why you just might not be getting the gas mileage you're suppose to.
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Oxygen Sensor
Oxygen sensors can eat 20% of your gas mileage, the sensor tells the (ECU) "electronic control unit" or lame man's terms a "car computer" to mix the proper amount of air to fuel mixture. If the O2 Sensor is bad it could be adding too much fuel,this can also lead to a bad Catalytic Converter eventually if not taken care of in time.

Spark Plugs & Wires
Spark Plugs & Wires are common problems with gas mileage, if they don't function properly by say not adding enough spark or a short in the wire, this can fool the air fuel mixture reading from the O2 Sensor causing poor gas mileage and other problems as well.Changing them can improve gas mileage and also smooth acceleration and deceleration along with over all performance.

Fuel Pressure Regulator
A stuck FPR aka "Fuel Pressure Regulator" can and will cause extremely bad gas mileage by not allowing the excess fuel to return to the fuel tank causing it to exit the fuel injectors. Other major problems can be caused from it including, burning out the fuel pump,hydro locking the engine & washing your piston rings.

Fuel Injector
A leaking fuel injector can cause poor drive ability, running rich will definitely kill gas mileage but, it can also dirty up your engines internals and wash your piston rings causing your cylinders to loose compression over time.Take the fuel injectors out and inspect them for leaks. A clogged injector can cause poor idle and even lean out cylinders causing engine failure.A good Gas Treatment can be necessary, you simply pour 1 bottle in the gas tank each time you fill up, drive your vehicle as you would and it does all the work by cleaning the injectors,valves and piston rings with caked on carbon. all of that can increase better fuel economy.

A/C On
A air conditioner can kill gas mileage also, if you can handle the heat, roll the windows down and save yourself some pocket change, unfortunately I'm not one of those pocket change savers and will keep my a/c on full blast! On the interstate it will not help due to too much drag.


Dirty Air Filter
More so then not, a dirty air filter can decrease your gas mileage by up to 10%!! Studies have shown that using a cold air intake can increase your mpg to about 5% better than if you didn't use one on your vehicle.Look for air filters labeled "HIGH FLOW". K&N Air Filters are the best on the market. They have the highest flow rating and are re-washable and re-usable, that makes them Number #1.

Low Tire Pressure
Freebie ALERT!:) Well..maybe a $1.00 at the local gas station. Low tire pressure can and will have a negative effect on gas mileage, lets face the facts, "you try pushing a full wheel barrel on a flat" It'll burn you out faster than a small sandwich can survive an eating competition. :D. I usually put 5lbs less then the max pressure implemented on the tire. Pick up a Tire Pressure Gauge and see if you are in the range that your tires recommend. All tire psi is located on the side wall of the tire.

Map/Mass Sensors
A dirty map sensor will richen the fuel mixture causing poor gas mileage and even a grayish tint smoke out the tail pipe. I tend to clean the MAP sensor with Lacquer Thinner for the more caked on carbon. As for the the Mass Air Flow Sensor, you will need a special solution called Mass Air Flow Cleaner. Basically you remove the Mass Air Flow Sensor from the intake manifold and spray the sensor with the cleaner until it's clean.

Idling Vehicle
This one has a record, "the, I'll leave my vehicle running because it uses less gas then to restart it ". Well this myth is a lie, you use way more gas idling then you would starting it, any gas that is not used in the starting process simply returns to the gas tank. When starting your vehicle, the injectors only spray the same amount of fuel as idle because the throttle is closed, it's telling the (TPS) "Throttle Positioning Sensor" to only spray so much fuel. If you idle for 5sec, and it takes you 5sec to start your car, you used the same amount of fuel. So in closing, turning your car off while waiting for someone saves you fuel. 

Engine OIL

Yes, Engine oil can and will eat your gas mileage, if you use the wrong weight, it could cost you upwards of 12% engine robbing $$CASH$$. The key is to use oils that have a higher efficiency of reducing friction, friction slows things down, what it's doing to your gas mileage, is slowly robbing you :D. Synthetic Oils are some of the best friction reducing oils. Stay within the same oil weight your manufacture spec's require. If your vehicle requires 10w20, don't put in 10w30 or you will loose mileage. NOTE: Increasing the oil weight won't hurt your engine,just your mileage.

Keeping Your RPM Down
It may sound like a no brainier, but keeping the rpm down will without a doubt give you better gas mileage. If you happen to be the person with the manual transmission that has 5-speed's, but always leaves it in 4th gear on a main road, well you're eating your gas mileage up. Put that baby in 5th and coast saving the tank. Driving slower does in fact save gas mileage, now the time that you arrive at your destination could greatly vary, at least you'll have a fuller tank doing so driving 55mph @ 2,500rpm vs 70mph @ 3,500rpm, the lower the rpm, the less gas used. 

Tire & Rim Type
If you're the big tire type guy/gal, then you're going to have to be the big spender type also. The more aggressive the tire, the more the mpg's drop. Yep, that goes for them muddin tires to. As far as rims go, a light aluminum rim will increase mpg compared to a heavier steel one.