The gas guzzler tax is an up front, federal charge if your car gets less than about 21 mpg, EPA average. It only applies to vehicles registered as cars, not trucks. In 2003 when we bought our minivan, I asked about registering it as a car instead of a truck. The dealer said they would collect the guzzler tank if I did I so. It’s a truck, for registration purposes, anyway.
@B.L.E, that’s an interesting question. I can’t say for sure, but avoiding the gas guzzler tax certainly hasn’t hurt SUV sales. I think the reason trucks avoided the tax is that many (most?) are work vehicles, and the tax was meant as a luxury tax, not a tax on working people. Determining whether a truck is a work vehicle or not is a lot harder than just eliminating trucks from the tax entirely. I do believe that the manufacturers recognized the tax gap and took advantage of it in bringing out new vehicles that customers liked a bought in big numbers.