what are the largest tires i am able to put on my disco's 255/55/18 rims without any rubbing or problems.

 

If 255/55's are what is stock size then you can up-size by 2.
So 275's for with and 65's for hieght.
Look on the drivers door jam for the tire sticker and it will tell what is the stock size, the tires that are on it may not be stock size so you must always go by the sticker.
But as a general rule no more than two sizes.

 

thank you very much