A very quick way, but not accurate, is to multiply the MPH number with 1.5
so if you see a vehicle running at 10MPH x 1.5 = 15KMH. It's not the most accurate, but it is very easy to get approximate speed in KMH without wasting time. The real answer to the above is a bit over 16km/h. Yes, multiplying by 1.6 would be more accurate, but not as easy to multiply on the fly.
That or do as I did and move to the UK for few years to get used to MPH.
The above is not an official reply