Why do US speed limits end in "5"?

Every other country, including our neighbor, Canada, has almost all speed limits end in "0." So, we get 30, 40, 50, 60, 70, etc. H0wever, in the US, they almost always end in "5," even though car speedometers show speeds ending in "0." I have often wondered why the US adopted this counter-intuitive and probably more difficult system. But like light switches going up to turn a light on, it is yet another example of American exceptionalism. I have tried to find the origins of this, but perhaps, like driving on the right or the left, there is no sure answer.