In the automotive industry, metric bolt heads and thread sizes have crept into the American cars over the years. If you go to the auto parts store to buy a bolt of a certain size, they'll ask you if it's Japanese metric or German metric, because there are different sizes used by each.
.
But when it comes to lug (bolt) wrenches for wheels and spark plug socket wrenches, as far as I have seen the standard remains the English system -- get this: even though in England they use metric today, not English.
.
The thread sizes of the lug bolts might be metric threads and the thread sizes of the spark plugs might be metric threads, but the wrenches used to tighten or loosen lug bolts and spark plugs remain in increments of 1/16 th of an inch.
.
For medical purposes, metric is much easier to use, partially due to the fact that one gram of water at 4 degrees Celsius is described as one cubic centimeter, about the size of a sugar cube. That determines the size of a meter, approximately, but in fact, the meter is defined more specifically by its relative size to one inch in the English system. It has been established that an inch is exactly equal to 2.54 centimeters (and therefore 0.0254 of a meter). Likewise, there are exactly 25.4 millimeters in an inch.
.
Tradesmen like carpenters, plumbers and electricians had to go through some painful adjustments when metric took over in Europe, and now today these tradesmen in America are facing a slow transition too, but it is not moving ahead very consistently here as it did in Europe. Tape measures are still in feet and inches and lumber sizes are still feet and inches and pipe and wire gauge sizes remain the same, in inches when appropriate and lengths measured in feet. I don't think you can find electrical wire sold by the meter anywhere in the USA, it's sold by the foot.
.
We have had a standard size for an inch for a much longer time, as machinists have used very precise inch measurements for about 200 years now. Therefore the centimeter (and meter) had to conform itself to the length of the inch already in universal usage.
.