Method and arrangement for achieving lower emissions during operation of a vehicle that includes comprising a combustion engine (10) that emits exhaust gases into an exhaust system (140) having a catalyzes (320). An injection device (310) is arranged to inject a substance into the exhaust system and a transmission (90) is provided that is driven by the engine. The method includes selecting a gear in the vehicle's transmission in response to at least information about whether catalytic conversion is required and information about the temperature (T) in the exhaust system so that the selected gear regulates the temperature in the exhaust system.