Most people know what is normal for their car when it comes to gas mileage. You may know approximately how many miles per gallon you get, or approximately how long a tank of gas should last with your normal driving conditions. However, if you notice that you are getting fewer miles per gallon or having to fill your tank up more frequently, you may find yourself wondering why this is. Here are a few of the common reasons why you may be getting fewer miles per gallon of gas.