I am about to buy some winter tyres for my tin-top (1.0 Corsa) , I have two steel wheels (one from an older Corsa , the other the spare from the current one)
Thing is - they look almost the same , they measure the same (width , offset , dia) , but the older one is marked 5J and the newer one is 6J.............whats all that about ? ? ?

thats the width of the rim

The Car Wheel bible is your friend, try Googling it.
5" or 6" is the width J is the pattern of the tyre bead location edge, I think

http://www.carbibles.com/tyre_bible_pg4.html

there you go

Thats it...................I realy should have gone to specsavers

Just been out to the garage and measured them both , and I couldnt find any difference in width..........so I had a closer look at the markings.............and what I thought was 6 , is just a slightly over-stamped 5......

Maybe Santa will bring me some glasses for Christmas

its possible that one is marked wrong, 5j would measure 5" between the flanges if they mesure the same then ones marked wrong.
Ray
oops a bit slow typing as usual

[Edited on 14/12/10 by RAYLEE29]

if you need any more Claire I have 4 off 5 inch corsa steelies laying about you can have if you want em FOC