275/65R18 = 32.075" diameter
305/65R18 = 33.610" diameter

4.75% difference

Even if they fit without rubbing, some issues you may not have considered:
1) You are likely going to have TCS issues
2) Your speedometer and odometer will be inaccurate unless you get the PCM programmed for the larger tires
3) Your off-the-line performance is going to suffer
4) Your gas mileage will also suffer