A method for operating a semi-automatic or automatic mechanical transmission (310) of a heavy truck when driving at idle speed. The method includes supplying fuel to the engine (300) 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 (S1) under engine-idle power. Depending upon traffic and environmental requirements which require a higher speed, the driver upshifts the semi-automatic or automatic transmission by depressing a control device (350) for manual gear selection arranged on a gear shift lever (340) of the truck and then drives the truck at a second substantially uniform driving velocity (S2) under engine-idle power. The second (S2) substantially uniform driving velocity is greater than the first (S1) substantially uniform driving velocity.
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