This study examines the relationship between neural activity and behavior in mice. We used spiking activity from the visual cortex, thalamus, hippocampus, and midbrain while mice performed a change detection task with water rewards. We analyzed the correlations between this spiking activity and the mice’s running speed. We found robust correlations between spiking activity and running speed across all brain regions, suggesting a widespread neural code for speed. The distribution of these correlations deviated from a simple linear model, implying a more complex relationship. Correlations were significantly stronger during active task periods (with reward) compared to passive periods, highlighting the modulatory role of reward in speed coding. Building on the observed strong correlations between neural activity and running speed, we explored the possibility of predicting the mouse’s speed using this spiking activity data. These findings reveal a more nuanced picture of speed coding in the brain, emphasizing the influence of reward and the involvement of a broader brain network beyond just visual processing areas.