Provided is a method for changing gear ratio in a gearbox comprising: receiving a signal to change the gear ratio; calculating braking torque that the engine should provide to reduce the rotational speed of the engine to a target rotational speed; phase-shifting a second camshaft in relation to the crankshaft, to a state where the exhaust valve is controlled to be opened during the expansion stroke and closed during the exhaust stroke; disconnecting the engine from the driving wheel; opening and closing the exhaust valve with a decompression device in a transition area between an exhaust stroke and an inlet stroke and also between a compression stroke and an expansion stroke, when the piston is at a top dead center in the cylinder to achieve engine braking through compression in the cylinders during the exhaust stroke and the compression stroke, and f) shifting a gear in the gearbox.