hi, right, the size of the rims is accademic as it is the tyres that really matter. if you have a 31 inch tall tyre then that iis how tall it is never mind the size of the rim supporting it. your 31/10.5/15 is the same height as my 750r16 or a 235/85r16 or a 265/75r16, though allowance must be made for different makes being measured differently by their makers. as for rrubbing diffs on the floor you are at about the max height a disco will stand without fairly drastic body chopping but if you want to go down that path then 35 inch tyres can be made to fit.