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1 gallon = 7.6 l, and anectada from Wikipedia (referenced to https://www.scientificamerican.com/article/strange-but-true-...) says 6 l in 3 hours killed a man. Half-life of caffeine is approximately 5 h, so quite similar.

1 gal is about half that (3.8 liters)

UK/imperial gallon is 4.5l US gallon is 3.78l more people used to use imperial than US but now most commonwealth economies are metric.

Fair enough, but it seems per clear that the OP was referring to US gal but the math was just off by double.