max Duster tire size

If you actually want them to fit inside the quarters and aren't moving the springs a 275/60/15 is probably the best you're going to do. Rolling the lips can buy you a little space, but at normal ride heights you also have to contend with the shape of the wheel tub, so even rolling the quarter lips only buys a little extra space before you start hitting the "bump" in the outer wheel tub.

I have 295's on my Duster with stock wheel tubs, but my springs are offset a 1/2" and I trimmed back the quarter lip a 1/2" too. And mine are maxed out, no extra clearance anywhere, even mine rubbed occasionally before I added a 7/8" rear sway bar.