I did not put the question clearly. I was thinking of altimeter settings changing on route. I suppose the regional settings would do most of the time. As SK says it works fine in the USA. However am I right thinking there is no IFR outside controlled airspace in the rest of Europe?
It might be interesting to do the sums for near my home base where three altimeter setting regions come together. There are occasionally dramatic differences in settings between Portree, Tyne and Belfast.
D.O.