Any system of measurement works as long as the guy making the other part also uses it. There is a big cost when one person uses their economic power to use one standard and everyone else uses another. Maybe when you finish off your economy properly we will be able to force metric upon you. Until then I guess we will buy two sets of tools and pray your pilots convert litres to pounds correctly. There isn't always a Canadian airport to catch the mistake ;-). Think how much cheaper your Chinese made in America Harley parts would have been if they used metric fasteners. Remember VHS vs Beta. Beta was better but the world chose VHS and even the most powerful consumer electronics company lost a lot of business in that war. You can be sure that even with it's world directing economic power that the US looses a lot of the world market by insisting on inches. Even on that specific scale 150mm sounds bigger than 6 inches. The adds would say add 50 by taking these little pills.