The bad news is, unless you can detect a real engine malfunction that a technician can find and deal with... you would likely waste time and money taking it in.
The good news is, it likely is not your car. The problem could be your gas. And in fact, likely is.
Have you changed brans of gasoline? Not all gas is the same. Some crappy brands are cheaper, but get worse gas mileage. If you did change, go back to the old brand, and see if mileage improves.
Have you changed what type of gas you use? Regular or Premium? Some people try to save money by buying regular, when their car needs premium, then complain about bad gas mileage. Do not do this. Buy the gas your car is designed for.
Finely, there are two things you can't change. In the winter, gas companies reformulate their fuels to deal with colder temperatures, like making gas more evaporative and less prone to water condensation. These added chemicals can reduce gas mileage.
Second, the amount of Ethanol mandated to be used in fuel was recently increased. Ethanol does not have as much energy as gas. Therefore more Ethanol in the fuel means less gas mileage.
Bottom line, unless you have a malfunction light coming on, or poor engine performance, stalling, misfires, hard starts, do not take your car in. Whatever problem will cost far more than worth to fix. This assumes the problem is your car and it likely is not.
So I have about a 14 gallon tank in my 94' Honda Accord. I reset my mile thingy every time i fill up. I would get about 360-370 miles per tank. All of a sudden, it just went to 340, and now i get 320 or less. Does anyone know of any reasons why this would happen so suddenly? I changed my spark plug wires and that didn't help. So i don't know what to do. I don't have much money and can't afford to take it in.

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