Mandatory Energy Conservation Act - Imposes a tax of 24 cents per gallon on gasoline sold by the producer or importer thereof. Terminates the Highway Trust Fund established under the Federal Aid Highway Act of 1959, covering any amounts in such fund into the general fund of the Treasury. Allows as a credit against the tax imposed by this Act 20 cents per gallon of gas purchased by the taxpayer during the taxable year up to $140 per year, reduced by 25 percent of the income of the taxpayer exceeding: (1) $12,000 in the case of an unmarried individual; (2) $15,000 in the case of a married individual filing a joint return; or (3) $7,500 in the case of a married individual filing a separate return.