On my 90GT with 17 x 9 Cobra Rs I could run 255/40R17 with quad shocks. I now run 275/40R17 without quad shocks. I went with MM LCAs and FMS UCAs and don't need the quads anymore. Anything wider than 275/40R17 would require a wider rim and wheel well work.
I have 17x9 all around (biggest you can go on the back) and i run nitto555's street radial 275/40/17 all around, front rubs on control arm but i can live with it and its my daily driver, and backs dont rub on quad shocks or anything