Motor Fuel Conservation Act of 1979 - Establishes the Motor Fuels Conservation Trust Fund within the Treasury of the United States. Appropriates to such Fund the amounts received by the Treasury from surtaxes on diesel fuel and gasoline. Directs the Secretary of the Treasury to: (1) hold such Fund; (2) report to Congress on the operations and financial conditions of such Fund; and (3) invest such portion of such Fund as is not required to meet current withdrawals. Stipulates that such investments be made only in interest-bearing obligations of the United States or in obligations guaranteed as to both principal and interest by the United States. Allocates to each State from such Fund an amount equal to the surtax on the sale of diesel fuel or gasoline within that State during each fiscal year. Stipulates that such amounts be: (1) returned to the residents of such State paying such surtax, to the extent practical; and (2) used to assist public mass transporation systems within such State. Imposes a surtax on the sale of diesel fuel and gasoline. Adjusts such tax for inflation. Makes such tax effective on sales after December 31, 1979.