many thanks, just did more diggin around and apparently the J is just some industry standard to do with the lips on the rims, doing a bit of research and have seen that for example on my rear 10J alloy 285/45 a43 fitted the 285 being 285mm ie 11.2inches same up front got 9j alloy with 255.55 r19 tire i.e 10 inch tire width, again here tires fitted to alloys are about 1inch wider than the alloy spec....dunno why?tighter fit, but for tire rotation purposes was thinking of getting 255/50 for rear also....255mm ie 10.02 inches is close to the size of the rear alloy width so dont see why this would be an issue as some say it is ...and some say it aint

if that makes any sense?