The purpose of this paper is to shed some light on the speed limit debate, and specifically to look into whether reducing speed by imposing a speed limit is better than achieving the same by imposing a bunker levy. This debate, along with the various issues of speed optimization versus speed reduction, is currently ongoing at the International Maritime Organization (IMO), in the quest to reduce greenhouse gas (GHG) emissions from ships. In that context, “speed optimization” and “speed reduction” have been included in the set of candidate short-term measures under discussion at the IMO. However, there is much confusion on what either speed optimization or speed reduction may mean, and some stakeholders have proposed mandatory speed limits as a measure to achieve GHG emissions reductions. To investigate this issue, the speed limit option is compared with the option of reducing speed via a bunker levy. The latter option is not under immediate discussion at the IMO, to be potentially included in the set of medium-term measures. The main result of the paper is that the speed limit option exhibits a number of deficiencies as an instrument aiming to reduce GHG emissions, at least vis-à-vis the bunker levy option.