Quote by jetwaterluffy Km/h isn't SI derived either (although a lot better than mph). The SI version of speed is m/s.
Really? What is "better" about it? Use units appropriate to the situation. If you're driving at noon, and want to know where you'll be at 5:00, then both kph or mph are "better" than m/sec. If we had decimal clocks I suppose that could change. But I don't see decimal clocks happening anytime soon.

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 Quote by gmax137 I'm also curious about this, does anyone anywhere say "I weigh 690 newtons" or does everyone say "...70 kilos" ?
I don't think so.

 At first I thought the units were funny (pounds per hour, gallons per minute, etc.) but soon they became familiar. As long as you understand what is going on, any system of units "works" and you may see the advantages of one or the other, depending on the given situation.
Imperial units have more conversion factors. If there is nothing to convert, it does not matter.

 Quote by gmax137 Really? What is "better" about it? Use units appropriate to the situation. If you're driving at noon, and want to know where you'll be at 5:00, then both kph or mph are "better" than m/sec. If we had decimal clocks I suppose that could change. But I don't see decimal clocks happening anytime soon.
Because from metres per second you can work out joules energy or newtons force straight away, so it makes more scientific sense, which is what the SI units were invented for. True, in a real life situation km/h is easier, but I'm just saying km/h isn't SI.