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There are literally dozens of potential causes for a decrease in fuel economy - many of them will also cause other symptoms with the way the vehicle operates, which can be helpful in diagnosing the problem. Some of the more common ones are:

* Fuel/Intake System Restrictions - Dirty air filters, fuel filters, and fuel injectors make your engine work harder to generate the same amount of power at the wheels, thus wasting more of its own energy and reducing mileage. This is probably the most common cause of gradual fuel economy loss because, unless the problem is severe, computer controls can usually compensate such that the average driver doesn't notice any issues. This is why it's important to stick to maintenance schedules, even if it seems unnecessary at the time.

* Engine Management Problems - Modern vehicles rely heavily on computer controls to "fine-tune" the engine in real time for maximum efficiency; an assortment of sensors monitor various engine parameters and feed data to the computer that allow it to make adjustments as conditions warrant. Most of these sensors are subjected to some "hostility" during operation (high temperatures, corrosive gases, contaminants, mechanical wear, etc) and do occasionally fail, preventing the computer from correctly doing its job and consequently reducing efficiency. Similarly, the computer in turn uses some electronic and electromechanical devices to actually control the engine, which can suffer the same fate. Problems of this type will almost always turn the Check Engine light on and cause additional driveability issues (rough idle, poor acceleration, stalling, etc).

* Wheel-End Problems - Low tire pressure is another common cause of low fuel mileage. Low pressure allows a greater portion of the tire tread to contact the road, increasing friction and sapping power (it also causes accelerated and abnormal tire wear). Wheel alignment problems cause similar issues by forcing tires to "drag" across the pavement to some degree, rather than roll smoothly. Less commonly, malfunctioning braking or all-wheel drive systems can cause additional drag at one or more wheels, wasting power.

* Environmental Issues - The environment and manner in which a vehicle is operated can have a big impact on its fuel economy that you might not consider unless you stop to think about it. Your mileage may decrease in snow, for example, because you spend more time in stop-and go conditions and in low gear, or in the summer months if you drive more aggressively. Changes in overall driving habits (going from mostly highway to lots of city driving) can also play a big role.

* "Parasitic" Problems - The engine is ultimately the source of all power used by every system in the vehicle, and as such, any part of the vehicle operating in an inefficient manner has the potential to reduce fuel economy. Clutch/transmission slippage, for example, can cause a dramatic loss in fuel economy, as can regenerative braking malfunctions in hybrid vehicles. Engine accessory problems (water pumps, alternators, etc) can put fuel-wasting drag on the engine, as can internal mechanical problems of the engine itself (these types of problems usually make themselves apparent in other ways - a bad alternator will cause electrical problems or make noise, for example). Even body damage or modifications can increase wind drag, decreasing fuel economy.

One last thing to keep in mind: it is entirely possible for several causes to simultaneously contribute slight effects which all add together to cause a more pronounced problem; hence, it isn't always possible to pinpoint a single "silver bullet" that will cure poor fuel economy, especially in higher-mileage vehicles.