a gallon of gas weighs

Gas prices hit a record high of $4.1144 a gallon on July 17th and then fell throughout the fall to hit a low of $1.6160 a gallon on December 30. The fall off in retail gas prices came on the back of sharply declining ...
http://rss.cnn.com/~r/rss/money_latest/~3/VuqcQ6MSBmo/index.htm

Looking back, the U.S. could have begun a gradual increase in the gas tax, starting with about $0.25 a gallon after the 1973 OPEC Embargo, increased another $0.25 after the 1979 Iranian Crisis and so on. ...
http://greeninc.blogs.nytimes.com/2009/01/14/will-a-mileage-tax-replace-the-gas-tax/