[folmar]

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.

[bumby]

1 gal is about half that (3.8 liters)

[ggm]

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.

[bumby]

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