You want a method of converting that is easy to use in all situations. The one I like goes like this:
100 km/h = 100 * 1000 m/3600 s = ....
You just replace the km with 1000m, and the hour with 3600 s.
This method always works and soon you don't even have to think about it. You can even do km/hr/minute or something weird like that.