If you are going to high load rated LT's or muddies,i would order five and maybe take a 19" tyre on rim for emergency,depending what size 18" you are going to run,(i know its not the right thing to do but in emergency would work),or just an 18" carcass.I have run LT's for the last ten years or so,and never changed one,done a heap of long trips on very rough roads in remote areas.But luck takes a part here as well.
You do well on 19",so you will probably never change an LT.
Rally experience probably helps with pressures,etc.....

