can i get some tire assistance please (sizing)

[pissonNOS]

ok im considering going to a set of 18's and im trying to figure out what size tire i would need to be compareable to what i have now, it could just be me but im having troubles trying to figure this out by looking at some size and spec sheets, I just used the ones from bf goodrich. Currently I am running 245-60-15's on the front which is 26.6 inches tall and 275-60-15's on the rear which is 28" tall can some one help me out here im kinda lost
Jason