depends on a variety of factors. Is your 90 standard weight? do you tow? carry heavy loads? motorway? go offroad? MT/AT or road bias tyres? etc. etc.
try 30f 35r for a start and see what you think. If its mostly road you could up it to 32f 40r. some people here run 35 all around. I vary mine to suit my needs, depends on the factors above amoungst others.
HTH
G