The federal government levies an excise tax on various motor fuels intended for highway use. Under current law, the tax rate is 18.3 cents per gallon on gasoline and 24.3 cents per gallon on diesel fuel. A 0.1 cents per gallon tax is also levied on top of these fuel tax rates to help fund expenses associated with fuel regulation. These rates do not automatically adjust for inflation. The statutory motor fuel excise tax rates were last increased in 1993. Specific tax rates also apply to special motor fuels, such as kerosene, compressed natural gas, and fuels derived from biomass. Motor fuels that are not intended for highway use are exempt from these taxes, as are specific federal government purchases of motor fuels. Under current law, federal motor fuels excise tax collections are credited to two federal spending accounts: the Highway Trust Fund (HTF) and the Leaking Underground Storage Tank (LUST) Trust Fund. The HTF is a federal transportation fund that is divided into two accounts: (1) mass transit and (2) highways. The HTF predominantly supports highway and transit programs and projects that are carried out at the state and local level. Although the HTF has other sources of revenue and is also credited with interest paid on the fund balances held by the U.S. Treasury, fuel taxes historically have provided almost 90% of the receipts to the HTF. The mass transit account receives 2.86 cents per gallon of fuel taxes, with the remainder of the tax revenue flowing into the highway account. The LUST Trust Fund receives 0.1 cents per gallon from the tax. Monies in the LUST Trust Fund are used to pay expenses incurred by the Environmental Protection Agency (EPA) and the states for preventing, detecting, and cleaning up leaks from petroleum underground storage tanks, as well as programs to evaluate the compatibility of fuel storage tanks with alternative fuels, methyl tertiary butyl ether (MTBE) compounds added to improve fuel combustion and air quality, and ethanol and biodiesel blends. Some Members of Congress have expressed concern that the HTF is no longer self-sustaining. In recent years, tax collections have declined largely because of inflation's effect on the real value of the tax rates, improvements in vehicle fuel efficiency, and less growth in vehicle miles traveled. Future declines in tax collections are projected largely due to scheduled increases in corporate average fuel economy (CAFE) standards . Since 2008, Congress has prevented projected shortfalls in the HTF by transferring money from the Treasury's general fund and the LUST Trust Fund to the HTF. While these transfers have been capable of fulfilling HTF obligations to the states temporarily, some Members of Congress have considered increasing excise tax rates or other revenue reforms to more closely align the growth of HTF receipts with the growth of obligations. Others, in contrast, have suggested cutting programs to reduce HTF outlays . This report provides an overview of current law and a legislative history of the federal motor fuels excise tax and financing of the HTF. Specifically, this report analyzes tax collections from gasoline and diesel motor fuels for highway use, since they have historically composed approximately 90% of the source of funding for the HTF. The report also includes a summary of the history of the gas tax and funding for the HTF in past sessions of Congress. The Appendix contains a legislative history of the gas tax dating back to its inception in 1932. Issues related to Federal Highway Trust Fund spending programs and infrastructure financing more broadly are beyond the scope of this report. Other CRS reports provide additional analysis of transportation finance issues: For more in-depth analysis of federal highway finance options (including the HTF), see CRS Report R42877, Funding and Financing Highways and Public Transportation , by [author name scrubbed] and [author name scrubbed]; and CRS Report R41869, The Donor-Donee State Issue in Highway Finance , by [author name scrubbed]. For an analysis of the economic effects of a change in the amount of the gasoline tax, see CRS Report R40808, The Role of Federal Gasoline Excise Taxes in Public Policy , by [author name scrubbed]. For information on transportation spending programs and infrastructure financing, see CRS Report R43582, Transportation, Housing and Urban Development, and Related Agencies (THUD): FY2015 Appropriations , by [author name scrubbed] and [author name scrubbed]; CRS Report R42966, Public Transportation Program and Funding Issues , by [author name scrubbed]; and CRS Report R43308, Infrastructure Banks and Debt Finance to Support Surface Transportation Investment, by [author name scrubbed] and [author name scrubbed]. Federal tax rates on motor fuels used for transportation purposes are summarized in Table 1 . Under current law, the federal tax on gasoline is 18.3 cents per gallon and diesel is taxed at 24.3 cents per gallon. A 0.1 cents per gallon tax is levied on top of these rates for the LUST Trust Fund. The rate of tax is 24.3 cents per gallon in the case of liquefied natural gas, any liquid fuel (other than ethanol or methanol) derived from coal, and liquid hydrocarbons derived from biomass. Other alternative motor fuels, such as compressed natural gas (CNG), are generally taxed at a rate of 18.3 cents per gallon. The current tax rates on gasoline and diesel motor fuels are scheduled to decrease to the permanent law rate of 4.3 cents per gallon (regardless of fuel type) after September 30, 2022. In addition, the 0.1 cent LUST Fund Tax is scheduled to expire on that same date. The role of the motor fuels excise tax has changed since its origination in 1932. Initially, the tax was among several deficit reduction tools. Increases in the "gas tax" were used in combination with other excise tax increases to finance emergency spending during wartime over the next two decades. In 1956, federal gasoline tax receipts were transferred to the newly created Highway Trust Fund. Federal gasoline excise tax rates were raised again in the early 1990s and directed partially for HTF funding purposes but also for general deficit reduction. In 1997, the portion of gas tax revenues directed to the general fund for deficit reduction in the early 1990s was redirected back to the HTF. The Appendix provides a more detailed history of the motor fuels excise tax and the HTF. Table 2 provides a complete summary of the gasoline tax rate changes since its establishment in 1932. Although not a focus of the analysis in this report, many state and local governments also levy their own motor fuel excise taxes. As such, changes to federal excise taxes will have an indirect impact on state and local motor fuel excise tax revenues. According to the American Petroleum Institute (API), a national trade association that represents the U.S. oil and natural gas industry, the average state excise tax rates on gasoline and diesel were 20.9 cents and 20.2 cents per gallon, respectively, as of January 2016. API also publishes variations of this analysis on a state-by-state basis. Motor fuel excise tax administration also varies by state. Some states have different rules for the point of taxation as some tax the product "at the rack," which is upon removal from the bulk terminal. In contrast, other states impose the tax at the distributor level where distributors hold licenses and file regular (usually monthly) returns with the state and local tax authority. By comparison, the majority of federal excise taxes are collected by the Internal Revenue Service (IRS) when the fuel is removed from the bulk storage terminals. The cumulative effect of federal, state, and local taxes contributes to some of the variation in fuel prices across the nation. The American Automobile Association (AAA) publishes daily average fuel prices in a particular state (and some metropolitan areas). Before 2008, the HTF was mostly self-sustaining, primarily through receipts generated by motor fuels excise taxes. Since 2008, the HTF has faced potential shortfalls in revenue and has required transfers in funding from the Treasury's general fund to meet its obligations to the states (under current law, the HTF cannot run negative balances). Some Members of Congress have expressed concerns regarding the state of the HTF and its ability to be self-financed given current trends in obligations and receipts. Several approaches to this shortfall have been discussed, both in hearings and more generally as part of the debate over funding for the HTF. For example, Congress has prevented potential shortfalls to the HTF by transferring over $139.9 billion (after sequestration) from Treasury's general fund to the HTF. As shown in Table 3 , general fund transfers to the HTF have been scheduled eight times by seven authorizing laws since 2008. Additionally, transfers totaling $3.7 billion have been scheduled from the LUST Trust Fund to the HTF. While ad hoc transfers in general funding would not enable the HTF to be self-sustaining, they could ensure that HTF obligations to the states are met through FY2020. As previously mentioned, the HTF cannot incur a negative balance under current law. Members have also discussed options to raise the federal gas tax as a means to offset the effects of inflation on the value of the tax rate and to compensate for other trends in motor transport (e.g., fewer passenger-car miles traveled and more fuel-efficient vehicles). Alternative tax regimes, or complements to the status quo motor fuel excise tax, such as a "vehicle miles traveled" (VMT) tax, have also been discussed as an option for addressing projected HTF shortfalls. On August 8, 2014, President Obama signed the Highway and Transportation Funding Act of 2014 ( P.L. 113-159 ).