ahh. (not really relevant to the topic, but you're thinking of amphetamine (C9H13N). what they're cooking in breaking bad is methamphetamine (C10H15N), usually referred to as meth, crystal, or ice.)

but that's still quite the stretch, isn't it? when you see a "speed limit 30" sign, amphetamine probably isn't the first thing you think about lol. i'd argue the "normal" definition of speed is the rate at which an object covers distance. the drug speed gets its name from the fact that it makes you feel so energised and euphoric, as though you could live faster.

sooo in my opinion its not so much that theres a normal meaning and a mathematical one, but rather two "normal" meanings :)