Well, lets see. A 235 is about 9 1/4", a 255 is 9 7/8" and a 275 is close to 10 3/4" wide. Lotta tire. So to step from a 235 to a 275 will require enough space for an extra 1" of tire on each side of your existing 235 combo. If you've got the inch, a 275 will fit.

The most backspace you can put under there with a 15" rim is 4.75. Beyond that you run into control arm interference. With a 17" rim, you could easily run 5 or 5.5 inches.