A method for operating a semi-automatic or automatic manual transmission of a heavy truck when driving at idle speed. The method includes supplying fuel to the engine of the heavy truck at a rate that facilitates engine-idle operation. In another step, the method engages the automatic or semi-automatic transmission in a gear higher than the starting gear of the transmission and permits the truck to operate at a first substantially uniform driving velocity under engine-idle power. Depending upon traffic and environmental requirements which require a slower or higher speed, the driver downshifts or upshifts the semi-automatic or automatic transmission by depressing a control pedal of the truck and then drives the truck at a second substantially uniform driving velocity under engine-idle power. For a downshift, the second substantially uniform driving velocity is less than the first substantially uniform driving velocity. For an upshift, the second substantially uniform driving velocity is greater than the first substantially uniform driving velocity.