Taxes on gasoline and diesel make up 85 percent of the revenue flowing into the United States' transportation spending account, but that may change.

Soon, the gas tax may not provide enough revenue to fund the nation's transportation spending, a new report from the Institute on Taxation and Economic Policy (ITEP) says.

One reason for plummeting gas tax revenue is the adoption of more fuel efficient cars, the report says.

Americans in their hybrids, electric cars, and other fuel efficient vehicles travel more miles on a given amount of gasoline--or in the case of electric cars, use none at all--meaning they have to buy less of it.

However, the report says the real culprit is the rising cost of road construction and maintenance.

While building and maintaining roads has gotten more expensive, the gas tax has remained the same. It was last raised 20 years ago today.

Gas pump

Gas pump

The authors of the report say the purchasing power of the gas tax has decreased by 28 percent since 1997.

They ascribe 22 percent of that decline to rising road construction costs, and 6 percent to increased fuel efficiency.

While most taxes are levied on a percentage basis, the gas tax is a fixed amount -- 18.4 cents per gallon -- and hence cannot adjust with inflation.

Road construction costs, however, do rise--and they have changed.

According to the report, the cost of building a road increased by 335 percent between 1972 and 2011.

This was lower than the general rate of inflation for consumer goods in the same period (438 percent), but still more than the current tax could be expected to fund.

On the other hand, vehicle fuel efficiency increased much less--66 percent--between 1972 and 2011, the report says.

To fix the problem, the report advocates raising the gas tax; the ITEP says it would have to be raised by at least 10 cents by 2015 to keep abreast of rising costs.

To prevent future discrepancies, the report also calls for allowing the gas tax to increase over time, based on average road construction costs and fuel economy increases.

As this story is published, the Federal government is preparing to shut down, based on the inability of the two political parties to reach a compromise to keep it in operation.

Is there a chance that the gas tax could be raised? If not, how should road repairs and construction be funded?

Read the full report here (PDF) and leave us your thoughts in the Comments below.


Follow GreenCarReports on Facebook, Twitter, and Google+.