The Gas Guzzler Tax which was part of Energy Tax Act of 1978. This tax was and still is imposed on manufacturers of cars that fail to meet a certain criteria of fuel economy levels. This tax only applies to cars; in other words, it does not apply to vans, SUVs and pick-up trucks. The tax wasn't applied to these vehicles because they were rather uncommon in this time period. They also were not taxed because these vehicles were most commonly used for business purposes rather than personal use. You can see how much tax if any the manufacturer had to pay by looking at the fuel economy window sticker of the car.

The criteria the IRS uses to determine the amount of tax is based on the vehicles fuel economy. The value used for this criteria is based 55%city driving and 45% highway. At the end of the year the car company must pay the a certain amount of tax based on how many vehicles were sold and their fuel economy rating.  The fuel economy tests are conducted by the EPA, in a laboratory, on a dynamometer which is a lot like a big treadmill.  

Since 1996 Ford has not had to pay the Gas Guzzler Tax on the Mustang.