I don't think this would be a good idea - the difference in circumference is nearly 3% - too much and it may damage your transfer case.

You might be better with 255/50-19 fronts and 275/45-19 rears. The difference between these two sizes is about 1%.

A 255 is normally fitted to an 8.5" rim but it will probably go on a 9" ok.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)