65 is the aspect ration, meaning it is 65% tall as it is wide. The first three number are your width and the last two are the hole. You probably have 215/65/15 on it right now and if you are changing rims to something wider you can get more tire under. 235/60/15 would be about as big as I think you can go, now if you would switch to a 16" rim....a 235/55/16 is what I would go with, yes it is still 235mm wide but having a lower "profile", the 55, it will give it a sportier look.
69 x-11..LS1, 420.88 horse to the tires,750 Holley Ultra DP, Torquer V2, darksides headers, PRC heads, 10.8:1 compression, forged bottom end, 5 speed. 2008 King Ranch F150 4x4.