Method and arrangement for regulating the rotational speed of a crankshaft included in an internal combustion engine during a gear changing process of a gearbox connected to the internal combustion engine. The arrangement includes a compressor mounted in connection with the air inlet of the internal combustion engine. A transmission having a variable gear ratio is located between the crankshaft and a driving shaft arranged in connection with the compressor, and a control unit is arranged to control the gear ratio between the crankshaft and the compressor. When changing from a higher to lower gear, the control unit is arranged to control the gear ratio between crankshaft and compressor so that the gear ratio between these components is reduced, and wherein moment of inertia is transferred from compressor to crankshaft resulting in an acceleration of the crankshaft.