A gas tax is basically regressive according to this paper (pdf page 9):

> The gasoline tax’s tendency to place a greater burden on low-income households is often stated as one of the strongest arguments against the tax [West, 2004a, 755]. Taxes on gasoline have been shown to be regressive in numerous economic studies. A 1989 study by Poterba concluded that gasoline taxes were about 15 percent of pre-tax income for the lowest income groups in the U.S., compared to 2.8 percent for the highest income groups [Poterba, 1989, 325].

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.554...