It's not just a matter of what the largest size will fit on the rim, but what will still allow clearance with regard to wheel openings, suspension components, etc. Just because a tire will fit the rim doesn't mean it will work on the car. Maybe it would help if you would explain what you are trying to accomplish.
Although 265/60R18 fits fine, but it is about 1.5 larger. Speedometer reads 60mph, actual speed is 63mph+. The scrub radius is negative -6. This may affect handling. The steering is probably lighter when turn left or right.