That does not answer Why? Trying to change the wheel system was attempted in the 80's but it didn't happen. Not only would all wheels become a different size but tyres would all have to be changed. Then 2 systems would have to be running as 99% of tyres run the current system, for instance 17" wheels would become 438.1mm which isn't a metric size, just a conversion so would become 440mm to most of us on here im sure mm is The same could be asked why do we buy petrol in ltrs and then worry about our miles per gallon And to throw in that we all buy a petrol can commonly referred to as a gallon can which holds 5l ... BUT a uk gallon is 4.54 litres ...